implement getgrouplist (for initgroups), formerly dummied-out

src/misc/getgrouplist.c

 #include <grp.h>
-
-/* FIXME */
+#include <string.h>
+#include <limits.h>
 
 int getgrouplist(const char *user, gid_t gid, gid_t *groups, int *ngroups)
 {
+	size_t n, i;
+	struct group *gr;
 	if (*ngroups<1) return -1;
-	*groups = gid;
+	n = *ngroups;
+	*groups++ = gid;
 	*ngroups = 1;
-	return 0;
+
+	setgrent();
+	while ((gr = getgrent()) && *ngroups < INT_MAX) {
+		for (i=0; gr->gr_mem[i] && strcmp(user, gr->gr_mem[i]); i++);
+		if (!gr->gr_mem[i]) continue;
+		if (++*ngroups <= n) *groups++ = gr->gr_gid;
+	}
+	endgrent();
+
+	return *ngroups > n ? -1 : *ngroups;
 }