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.

src/passwd/fgetgrent.c

@@ -0,0 +1,9 @@
+#include "pwf.h"
+
+struct group *fgetgrent(FILE *f)
+{
+	static char *line, **mem;
+	static struct group gr;
+	size_t size=0, nmem=0;
+	return __getgrent_a(f, &gr, &line, &size, &mem, &nmem);
+}