Make sure we are not running several instances.

idea from otto@openbsd

openup

@@ -95,10 +95,11 @@ _TMPDIR=$(mktemp -dp ${_TMP} .openup-XXXXXXXXXX) || exit 1
 _TMPKEY="${_TMPDIR}/key"
 _TMPSPLITCERT="${_TMPDIR}/cert"
 _TMPVUXML="${_TMPDIR}/vuxml"
+_PID="/var/run/${0##*/}.pid"
 
 export PKG_PATH=${PKG_PATH_UPDATE}:${PKG_PATH_MAIN}
 
-trap "rm -rf ${_TMPDIR}" 1 2 3 13 15
+trap "rm -rf ${_TMPDIR} ${_PID}" 1 2 3 13 15
 
 pr_bigarrow() {
 	if [ -z "${checkmode}" ]; then
@@ -303,6 +304,13 @@ if [ "${checkmode}" ]; then
 	fi
 fi
 
+if [ -f ${_PID} ]; then
+	pr_error "openup is already running ($(cat ${_PID})):"
+	pr_error "${_PID}"
+	exit 1
+fi
+echo $$ >${_PID}
+
 check_openupd
 
 if [ -n "${exclude_kernel}" ]; then
@@ -330,3 +338,4 @@ fi
 do_i_need_to_reboot
 
 rm -rf ${_TMPDIR}
+rm ${_PID}