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 various -Wpointer-compare warnings

This warning (comparing a pointer against a zero character literal
rather than NULL) has existed since GCC 7.1.0, and was recently added to
Clang trunk.

Almost all of these are harmless, except for fwcontrol's str2node, which
needs to both guard against dereferencing a NULL pointer (though in
practice it appears none of the callers will ever pass one in), as well
as ensure it doesn't parse the empty string as node 0 due to strtol's
awkward interface.

Submitted by: James Clarke <jtrc27@jrtc27.com>
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21914

getserv_test: mark unused parameters __unused to fix corresponding
warnings

MFC after: 3 days
Sponsored by: Dell EMC Isilon

getserv_test: fix -Wsign-compare and -Wmissing-prototypes warnings

MFC after: 3 days
Sponsored by: Dell EMC Isilon

lib/libc/tests/nss: use calloc appropriately

The pattern used prior to this commit was `calloc(1, n * sizeof(type))`;
the pattern that should be used however is `calloc(n, sizeof(type))`.

MFC after: 3 days
Sponsored by: Dell EMC Isilon

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

- Convert the testcases to ATF
- Do some style(9) cleanups:
-- Sort headers
-- Apply indentation fixes
-- Remove superfluous parentheses
- Explicitly print out debug printfs for use with `kyua {debug,report}`; for
items that were overly noisy, they've been put behind #ifdef DEBUG
conditionals
- Fix some format strings

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