move strftime_l into strftime.c and add __-prefixed version

the latter is both for ABI purposes, and to facilitate eventually
adding LC_TIME support. it's also nice to eliminate an extra source
file.

src/locale/strftime_l.c

@@ -1,7 +0,0 @@
-#include <locale.h>
-#include <time.h>
-
-size_t strftime_l(char *restrict s, size_t n, const char *restrict f, const struct tm *restrict tm, locale_t l)
-{
-	return strftime(s, n, f, tm);
-}