libc: remove gets

gets is unsafe and shouldn't be used (for many years now). Leave it in
the existing symbol version so anything that previously linked aginst it
still runs, but do not allow new software to link against it.

(The compatability/legacy implementation must not be static so that
the symbol and in particular the compat sym gets@FBSD_1.0 make it
into libc.)

PR: 222796 (exp-run)
Reported by: Paul Vixie
Reviewed by: allanjude, cy, eadler, gnn, jhb, kib, ngie (some earlier)
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12298