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/

print_positional_test: Fix misuse of wchar APIs

These APIs take unit length, not byte length parameters.

Reported by: Coverity
CIDs: 1338543, 1338544, 1338545
Sponsored by: EMC / Isilon Storage Division

Convert print_positional_test over to ATF

Somehow missed in r290537

X-MFC with: r290537
MFC after: 1 week
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