Correctly handle small MAP_STACK requests.

If mmap(2) is called with the MAP_STACK flag and the size which is
less or equal to the initial stack mapping size plus guard,
calculation of the mapping layout created zero-sized guard. Attempt
to create such entry failed in vm_map_insert(), causing the whole
mmap(2) call to fail.

Fix it by adjusting the initial mapping size to have space for
non-empty guard. Reject MAP_STACK requests which are shorter or equal
to the configured guard pages size.

Reported and tested by: Manfred Antar <null@pozo.com>
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week