Simplify anonymous memory handling with an OBJ_ANON flag. This eliminates

Simplify anonymous memory handling with an OBJ_ANON flag.  This eliminates
reudundant complicated checks and additional locking required only for
anonymous memory.  Introduce vm_object_allocate_anon() to create these
objects.  DEFAULT and SWAP objects now have the correct settings for
non-anonymous consumers and so individual consumers need not modify the
default flags to create super-pages and avoid ONEMAPPING/NOSPLIT.

Reviewed by:	alc, dougm, kib, markj
Tested by:	pho
Differential Revision:	https://reviews.freebsd.org/D22119