Consider re-enabling Cirrus CI for testing on FreeBSD

[patmaddox]

Open questions

• Is it desirable to broadly pursue FreeBSD compatibility? For my purposes, the only component I'm interested in is termios, because Tailscale uses it (see Add tailssh support on FreeBSD tailscale/tailscale#6155).
• If so, how should we approach it? I propose that we start by simply not building the failing tests on FreeBSD. That would provide a TODO list of components to work on for FreeBSD support.

I would be happy to try an initial pass, using the proposed approach above. I suspect at least a category of the failures are due to Linux-specific unix.* var names. One example of addressing them in a single component is at #2544.

Context

From what I can tell, CI doesn't currently verify FreeBSD.

#2421 says:

Remove cirrus ci: It was only used for verifying freebsd buildability. We are going to use github actions for that instead (#2419)

GitHub actions doesn't officially support FreeBSD as far as I know. The method I'm aware of requires you to run a FreeBSD VM inside of a MacOS instance. That's pretty janky.

Cirrus CI runs directly on FreeBSD machines, so it should probably be re-instated for FreeBSD verification. I am happy to help with configuring it.

In #2544 (review), @rminnich suggested it would be worthwhile to bring back Cirrus CI for FreeBSD verification.

As an initial investigation, I tried running the u-root test suite on FreeBSD 13.1. Here are the failures:

--- FAIL: TestPackageRewriteFile (0.19s)
FAIL
FAIL	github.com/u-root/u-root/pkg/bb	0.202s
FAIL	github.com/u-root/u-root/pkg/boot [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/bls [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/bootcmd [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/boottest [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/esxi [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/fit [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/grub [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/jsonboot [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/localboot [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/menu [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/multiboot [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/netboot [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/netboot/ipxe [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/netboot/pxe [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/netboot/simple [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/purgatory [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/syslinux [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/systembooter [build failed]
FAIL	github.com/u-root/u-root/pkg/boot/uefi [build failed]
FAIL	github.com/u-root/u-root/pkg/checker [build failed]
FAIL	github.com/u-root/u-root/pkg/cmos [build failed]
FAIL	github.com/u-root/u-root/pkg/crypto [build failed]
FAIL	github.com/u-root/u-root/pkg/dhclient [build failed]
FAIL	github.com/u-root/u-root/pkg/dt [build failed]
FAIL	github.com/u-root/u-root/pkg/efivarfs [build failed]
--- FAIL: TestPassFD (0.00s)
--- FAIL: TestBadNonce (0.00s)
--- FAIL: TestBadSubsetNonce (0.00s)
--- FAIL: TestBadEmptyNonce (0.00s)
--- FAIL: TestTimeoutDoesntFire (0.00s)
--- FAIL: TestTimeoutFires (0.00s)
--- FAIL: TestWaitTimeout (0.00s)
--- FAIL: TestMultiServe (0.00s)
FAIL
FAIL	github.com/u-root/u-root/pkg/fdsrv	0.010s
FAIL	github.com/u-root/u-root/pkg/flash [build failed]
FAIL	github.com/u-root/u-root/pkg/flash/sfdp [build failed]
FAIL	github.com/u-root/u-root/pkg/ipmi [build failed]
FAIL	github.com/u-root/u-root/pkg/ipmi/ocp [build failed]
--- FAIL: TestLdd (0.00s)
--- FAIL: TestLddList (0.00s)
FAIL
FAIL	github.com/u-root/u-root/pkg/ldd	0.011s
FAIL	github.com/u-root/u-root/pkg/memio [build failed]
FAIL	github.com/u-root/u-root/pkg/mount [build failed]
FAIL	github.com/u-root/u-root/pkg/mount/block [build failed]
FAIL	github.com/u-root/u-root/pkg/mount/mtd [build failed]
FAIL	github.com/u-root/u-root/pkg/pci [build failed]
FAIL	github.com/u-root/u-root/pkg/pty [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch/eventlog [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch/launcher [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch/measurement [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch/policy [build failed]
FAIL	github.com/u-root/u-root/pkg/securelaunch/tpm [build failed]
FAIL	github.com/u-root/u-root/pkg/smbios [build failed]
FAIL	github.com/u-root/u-root/pkg/strace [build failed]
FAIL	github.com/u-root/u-root/pkg/strace/internal/abi [build failed]
FAIL	github.com/u-root/u-root/pkg/termios [build failed]
FAIL	github.com/u-root/u-root/pkg/tss [build failed]
FAIL	github.com/u-root/u-root/pkg/txtlog [build failed]
FAIL	github.com/u-root/u-root/pkg/uefivars/boot [build failed]
--- FAIL: TestCreateInitramfs (3.61s)
    --- FAIL: TestCreateInitramfs/Test_0_[BB_archive_with_ls_and_init] (1.39s)
    --- FAIL: TestCreateInitramfs/Test_5_[multi-mode_archive] (1.68s)
FAIL
FAIL	github.com/u-root/u-root/pkg/uroot	3.643s
FAIL	github.com/u-root/u-root/pkg/watchdog [build failed]
FAIL	github.com/u-root/u-root/pkg/watchdogd [build failed]