add fgetgrent function

based on patch by Isaac Dunham, moved to its own file to avoid
increasing bss on static linked programs not using this nonstandard
function but using the standard getgrent function, and vice versa.

include/grp.h

@@ -30,6 +30,10 @@ struct group  *getgrent(void);
 void           endgrent(void);
 void           setgrent(void);
 
+#ifdef _GNU_SOURCE
+struct group  *fgetgrent(FILE *stream);
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 int getgrouplist(const char *, gid_t, gid_t *, int *);
 int setgroups(size_t, const gid_t *);