MFC r267546 (by alc):
Tidy up the early parts of vm_map_insert().

MFC r267645 (by alc):
When MAP_STACK_GROWS_{DOWN,UP} are passed to vm_map_insert() set the
corresponding flag(s) in the new map entry.
Pass MAP_STACK_GROWS_DOWN to vm_map_insert() from vm_map_growstack() when
extending the stack in the downward direction.

MFC r267850 (by alc):
Place the check that blocks map entry coalescing on stack entries in
vm_map_simplify_entry().

MFC r267917 (by alc):
Delay the call to crhold() in vm_map_insert() until we know that we won't
have to undo it by calling crfree().
Eliminate an unnecessary variable from vm_map_insert().

MFC r311014:
Style fixes for vm_map_insert().

Tested by: pho