strongswan: mark proposals and esp_proposals as optional

The proposals (IKE_SA) and esp_proposals (CHILD_SA) settings in
swanctl.conf do not have to be configured explicitly. If omitted,
charon falls back to its default proposals:

 - IKE: the default proposals include all algorithms supported by the
   loaded plugins that are not considered insecure.
 - ESP: the default proposals are hard-coded, as the kernel can't be
   queried for its supported algorithms. They include the supported
   key exchange methods as well as "none", which makes PFS optional.

The default ESP proposal is only used if neither ah_proposals nor
esp_proposals are configured for a CHILD_SA.

Update the config to reflect that both options are optional.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/strongswan/files/swanctl.init

@@ -390,7 +390,7 @@ config_child() {
 	[ -n "$if_id" ] && swanctl_xappend4 "if_id_in = $if_id" "if_id_out = $if_id"
 	[ -n "$start_action" -a "$start_action" != "none" ] && swanctl_xappend4 "start_action = $start_action"
 	[ -n "$close_action" -a "$close_action" != "none" ] && swanctl_xappend4 "close_action = $close_action"
-	swanctl_xappend4 "esp_proposals = $esp_proposal"
+	[ -n "$esp_proposal" ] && swanctl_xappend4 "esp_proposals = $esp_proposal"
 	swanctl_xappend4 "mode = $mode"
 
 	if [ -n "$life_time" ]; then
@@ -726,7 +726,7 @@ config_connection() {
 	fi
 	[ -n "$over_time" ] && swanctl_xappend2 "over_time = $over_time"
 
-	swanctl_xappend2 "proposals = $ike_proposal"
+	[ -n "$ike_proposal" ] && swanctl_xappend2 "proposals = $ike_proposal"
 	[ -n "$dpd_delay" ] && swanctl_xappend2 "dpd_delay = $dpd_delay"
 	[ $encap -eq 1 ] && swanctl_xappend2 "encap = yes" || swanctl_xappend2 "encap = no"
 	[ "$keyingtries" = "%forever" ] && swanctl_xappend2 "keyingtries = 0" || swanctl_xappend2 "keyingtries = $keyingtries"