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/

Fix double fclose of `fp1` when freopen fails

freopen handles closing file descriptors on error, with the exception of
fdopen'ed descriptors, so closing an already fclose'd file descriptor is
incorrect

CID: 1338525
Differential Revision: https://reviews.freebsd.org/D6013
MFC after: 2 weeks
Reported by: Coverity
Sponsored by: EMC / Isilon Storage Division

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

- Fix some whitespace
- Convert the testcases to ATF
- Convert "/dev/null" to _PATH_DEVNULL

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