libc: Add strverscmp(3) and versionsort(3)

Add a strverscmp(3) function to libc, a GNU extension I implemented by
reading its glibc manual page. It orders strings following a much more
natural ordering (e.g. "ent1 < ent2 < ent10" as opposed to
"ent1 < ent10 < ent2" with strcmp(3)'s lexicographic ordering).

Also add versionsort(3) for use as scandir(3)'s compar argument.

Update manual page for scandir(3) and add one for strverscmp(3).

Reviewed by: pstef, gbe, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35807