libsa: Remove redundant sys/cdefs.h

Sponsored by: Netflix

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Move stand/ofw/libofw to stand/libofw.

Since rS330365, there has been no particular reason for libofw to be in a
subdirectory of ofw. Move libofw up a level to make it fit in better with
the other top level libraries.

Also add a LIBOFWSRC to stand/defs.mk to match what all the other
libraries are doing.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D23000