libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

Remove $FreeBSD$: one-line .c pattern

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

Remove unused variables; sort by alignment where needed

MFC after: 1 week
X-MFC with: r290532
Sponsored by: EMC / Isilon Storage Division

Integrate tools/regression/lib/libc/locale into the FreeBSD test suite
as lib/libc/tests/locale

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division