Add some checks on domjudge_user and runuser.

They should not be the same, and neither should be equal to
root, but definitely runuser should not be as it is the
unprivileged user that submissions are run as.

For domjudge_user there is already a (overridable) check
that it is not root.

(cherry picked from commit d9c298f)

Author: eldering

configure.ac

@@ -117,6 +117,13 @@ else
 	AC_MSG_RESULT($RUNUSER)
 fi
 
+if test "x${DOMJUDGE_USER#"$RUNUSER"}" != "x$DOMJUDGE_USER" ; then
+	AC_MSG_ERROR([domjudge_user '$DOMJUDGE_USER' cannot match runuser '$RUNUSER'.])
+fi
+if test "x$RUNUSER" = "xroot" ; then
+	AC_MSG_ERROR([runuser cannot be root.])
+fi
+
 AC_MSG_CHECKING([rungroup])
 AC_ARG_WITH([rungroup], [AS_HELP_STRING([--with-rungroup=GROUP],
 [Unprivileged group under which to run submissions (default: same as runuser).])], [], [])