Document flags of vm_page allocation functions.

Reviewed by:	alc
Do not clear flag that vm_page_alloc() doesn't support.

Submitted by:	kib
Add flag VM_ALLOC_NOWAIT for vm_page_grab() that prevents sleeping and
allows the function to fail.

Reviewed by:	kib, alc
Sponsored by:	Nginx, Inc.
Merge remote-tracking branch 'freebsd/master' into hardened/current/master