Add vm_map_insert1(9)

The function returns the newly created entry.
Use vm_map_insert1() in stack grow code to avoid gap entry re-lookup.

The comment update for vm_map_try_merge_entries() was suggested by dougm.

Suggested by: alc
Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41099