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/

Polish up the tests a bit more after projects/collation was merged to head

Provide more meaningful diagnostic messages if LC_CTYPE can't be set properly
instead of segfaulting, because setlocale returns NULL and strcmp(NULL, b) will
always segfault

Split up the testcases so one failing (in this case en_US.ISO8859-15) won't
cause the rest of the testcases to be skipped

Remove some unused variables

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

Delete leftover printfs from when these were TAP tests

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