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/

Conform to Berne Convention.

Prompted by: Recent discussion
MFC after: 3 days

Add new gets_s(3) stdio function.

This implements the gets_s(3) function as documented at
http://en.cppreference.com/w/c/io/gets. It facilitates the
optional removal of gets(3).

Reviewed by: ed
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D12785