stdio: provide _unlocked variants of fflush, fputc, fputs, fread, fwrite

fflush_unlocked is currently desired in ports by sysutils/metalog, and
redefined as the locked fflush.

fputc_unlocked, fputs_unlocked, fread_unlocked, and fwrite_unlocked are
currently desired in ports by devel/elfutils, and redefined as the locked
fputs, fread, and fwrite respectively.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D23336