Port t_kevent to FreeBSD

Submitted by: pho
Port t_mincore to FreeBSD

Mark :mincore_resid as atf_tc_expect_fail on FreeBSD because of new bug
discovered in running the tests (it succeeded from earlier on in the year to
September/October on FreeBSD, at least)

Submitted by: pho
Port h_atexit to FreeBSD

__cxa_atexit varies between FreeBSD and NetBSD, and thus we must use pointers
instead of static fields in the BSS. More extensive discussion is included in
the source code

In collaboration with: kib
Submitted by: pho
Expect :snprintf_posarg_error to blow up with a SIGSEGV on !NetBSD OSes
Disable testcases 12 and 15-22 on FreeBSD

Submitted by: pho