user-util: add new helper fully_set_uid_gid()

Usually when we do setresuid() we also do setesgid() and setgroups().
Let's add a common helper that does all three, and use it everywhere.

src/basic/user-util.c

@@ -661,17 +661,26 @@ int get_shell(char **ret) {
         return path_simplify_alloc(e, ret);
 }
 
-int reset_uid_gid(void) {
+int fully_set_uid_gid(uid_t uid, gid_t gid, const gid_t supplementary_gids[], size_t n_supplementary_gids) {
         int r;
 
-        r = maybe_setgroups(0, NULL);
+        assert(supplementary_gids || n_supplementary_gids == 0);
+
+        /* Sets all UIDs and all GIDs to the specified ones. Drops all auxiliary GIDs */
+
+        r = maybe_setgroups(n_supplementary_gids, supplementary_gids);
         if (r < 0)
                 return r;
 
-        if (setresgid(0, 0, 0) < 0)
-                return -errno;
+        if (gid_is_valid(gid))
+                if (setresgid(gid, gid, gid) < 0)
+                        return -errno;
 
-        return RET_NERRNO(setresuid(0, 0, 0));
+        if (uid_is_valid(uid))
+                if (setresuid(uid, uid, uid) < 0)
+                        return -errno;
+
+        return 0;
 }
 
 int take_etc_passwd_lock(const char *root) {

src/basic/user-util.h

@@ -57,7 +57,10 @@ int getgroups_alloc(gid_t** gids);
 int get_home_dir(char **ret);
 int get_shell(char **ret);
 
-int reset_uid_gid(void);
+int fully_set_uid_gid(uid_t uid, gid_t gid, const gid_t supplementary_gids[], size_t n_supplementary_gids);
+static inline int reset_uid_gid(void) {
+        return fully_set_uid_gid(0, 0, NULL, 0);
+}
 
 int take_etc_passwd_lock(const char *root);