sys: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .h pattern

Remove /^\s*\*+\s*\$FreeBSD\$.*$\n/

sys: Extract __bitcount* from sys/types.h to new sys/bitcount.h

This will allow the code to be reused by the cross-build sys/types.h
wrapper in order to provide the APIs for greater compatibility. This
also provides a path towards eventually removing the definitions from
sys/types.h altogether if so desired by gradually migrating users to
including sys/bitcount.h explicitly, but that is not the primary goal
here.

Note that the copyright header is a direct copu of sys/types.h's given
that's where this code comes from. This could be replaced in future with
a more specific one restricted to just the code in question, depending
on what the copyright for that code is.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D36132