Add _NEW flag to mtx(9), sx(9), rmlock(9) and rwlock(9).
A _NEW flag passed to _init_flags() to avoid check for double-init.

Differential Revision:	https://reviews.freebsd.org/D1208
Reviewed by:	jhb, wblock
MFC after:	1 Month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275751 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f