tree-wide: use {pid,uid,gid}_is_valid() where appropriate

Also, drop UID/GID validity checks from getpeercred() as the kernel will
never pass us invalid UID/GID on userns, but the overflow UID/GID
instead. Add a comment about this.

src/basic/socket-util.c

@@ -967,15 +967,13 @@ int getpeercred(int fd, struct ucred *ucred) {
         if (n != sizeof(struct ucred))
                 return -EIO;
 
-        /* Check if the data is actually useful and not suppressed due
-         * to namespacing issues */
-        if (u.pid <= 0)
-                return -ENODATA;
-        if (u.uid == UID_INVALID)
-                return -ENODATA;
-        if (u.gid == GID_INVALID)
+        /* Check if the data is actually useful and not suppressed due to namespacing issues */
+        if (!pid_is_valid(u.pid))
                 return -ENODATA;
 
+        /* Note that we don't check UID/GID here, as namespace translation works differently there: instead of
+         * receiving in "invalid" user/group we get the overflow UID/GID. */
+
         *ucred = u;
         return 0;
 }