Fix FreeBSD build break

[Thefrank]

sys/extattr.h was failing check_include_file_cxx as it needs an addition header to build.
This also rolls PR #17 into this PR as it fixes some GCC-isms in the BSD section.

[Thefrank]

Other notes while I wait for checks:
Tests only support netcoreapp3.1 and net6 which are both end of service. To run the tests, I had to make changes which are not committed here (e.g., move to net8, override warnings, add FreeBSD to build.sh)

Two test failures that I would like to see if I can fix before removing WIP.
Test report in both trx and html are zipped and attached for those wanting to take a look.

Mono.Unix.Tests_net8.0_x64.zip

[akoeplinger]

Tests only support netcoreapp3.1 and net6 which are both end of service.

Yeah we can move tests to net8.0

[Thefrank]

This should be enough for now. It unbreaks FreeBSD but does not make any other change that might be breaking like moving tests to net8. The ipv6 test failure appears to be environment and how FreeBSD exposes information into jails. The other failure, fails the same way on OSX.

Github still lacks runners for FreeBSD and the current workarounds for getting FreeBSD into a Github runner significantly add to test run times.