Explicitly set MAP_PRIVATE to avoid [EINVAL] after r271635.

X-MFC-With:	r271635
Reviewed by:	jhb
The vm_mmap_cdev() explicitely converts absence of both MAP_SHARED and
MAP_PRIVATE flags to MAP_SHARED.  Apparently, some code in tree, in
particular, libgeom, relied on this behaviour, see r271721.  For
regular file types, the absence of the flags is interpreted as
MAP_PRIVATE, and libc nlist used this (fixed in r271723).

Allow the implicit flags for legacy binaries.  Bump __FreeBSD_version
to get the ABI note on new binaries to check for in mmap code.

Remove the test for presence of one of the MAP_ANON, MAP_SHARED or
MAP_PRIVATE flags before fget_mmap().  For MAP_ANON, we already verify
that passed fd == -1.  For fd != -1, test after fget_mmap() (for newer
binaries) covers the case.

Reported by:	bdrewery, pho
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
Merge remote-tracking branch 'freebsd/master' into hardened/current/master