When MAP_STACK_GROWS_{DOWN,UP} are passed to vm_map_insert() set the
corresponding flag(s) in the new map entry.  Previously, the caller was
responsible for setting them after vm_map_insert() returned.

Pass MAP_STACK_GROWS_DOWN to vm_map_insert() from vm_map_growstack() when
extending the stack in the downward direction.

Together these changes slightly simplify the caller's task when creating a
downward growing stack.  In particular, the caller no longer needs to clip
the previous entry, because the new stack entry can't possibly coalesce
with the previous entry.

Reviewed by:	kib
Sponsored by:	EMC / Isilon Storage Division