Only call chown if our effective userid is root

See just-containers/s6-overlay#309

src/s6-overlay-preinit/s6-overlay-preinit.c

@@ -43,21 +43,27 @@ int main (void)
     }
   }
 
-  /* requirement: /var/run/s6 must be owned by current user */
+  /* requirement: /var/run/s6 must be owned by current user,
+   * if (and only if) current user != root */
+  uid_t uid  = getuid() ,
+        gid  = getgid() ,
+        euid = geteuid() ;
+
   struct stat s6stat ;
   if(stat(VAR_RUN_S6, &s6stat) == -1)
   {
     strerr_diefu2sys(111, "stat ", VAR_RUN_S6) ;
   }
 
-  uid_t uid = getuid() ,
-        gid = getgid() ;
   /* only call chown if uid/gid are not from current user */
   if(s6stat.st_uid != uid || s6stat.st_gid != gid) 
   {
-    if (chown(VAR_RUN_S6, uid, gid) == -1)
+    if (euid == 0) /* chown will fail if we're not root */
     {
-      strerr_diefu2sys(111, "chown ", VAR_RUN_S6) ;
+      if (chown(VAR_RUN_S6, uid, gid) == -1)
+      {
+        strerr_diefu2sys(111, "chown ", VAR_RUN_S6) ;
+      }
     }
   }