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/

lib/libc/tests/iconv: raise WARNS to 6

MFC after: 2 weeks

Remove 'All rights reserved' from my files

See r333391 for the rationale.

Approved by: emaste (for the Foundation copyright)
Sponsored by: Dell EMC

iconvctl(3): remove superfluous NULL pointer tests

convname and dst are guaranteed to be non-NULL by iconv_open(3).
src is an array. Remove these tests for NULL pointers.
While I'm here, eliminate a strlcpy with a correct but suspicious-looking
calculation for the third parameter (i.e. not a simple sizeof).
Compare the strings in-place instead of copying.

Found by: bdrewery
Found by: Coverity
CID: 1130050, 1130056
MFC after: 3 days
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D6338