MFC r331936, r331942, r331943, r331945, r331947, r331948

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
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D12785