Regressed by D23696.
On FreeBSD < 13 neither `memfd_create` exists nor `posix_fallocate` works
with file descriptors returned by shm_open(). Given SHM_ANON code
wasn't upstreamed just undefine it`shm_open`. No need to check FreeBSD version
as SHM_ANONAs `SHM_ANON` code wasn't upstreamed and is not used on FreeBSD >= 1313 just disable `posix_fallocate` without version checks.