Make t_msync pass

The new (and correct per-POSIX) requirement is that any unmapped pages
fed into msync should return ENOMEM, not EFAULT. NetBSD's manpage says
this as well, so it's a NetBSD bug that was hidden by a test that was
validating an incorrect requirement.

Use ATF_REQUIRE_ERRNO instead of handrolled ATF_REQUIRE calls.

Test for EINVAL after feeding in invalid flags.
Add some TODOs