libc: Fix fmemopen(3) prototype in fopen(3) man page.

While here, also update a mention of ANSI C.

Sponsored by: Klara, Inc.
Reviewed by: kevans, markj
Differential Revision: https://reviews.freebsd.org/D41686

(cherry picked from commit 5a57401e7106132b61b16e34365cebf52b773007)

libc: Further nit in fopen(3) man page.

Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41687

(cherry picked from commit c9f5889d05b5854be033849a4ff9985699071548)