Add option to call external command on port change.

Add support for the PF_POST_COMMAND environment variable in
port_forwarding.sh. This will cause an external command to be called
with the new forwarding port and the original forwarding port (as
command-line arguments) every time the forwarding port changes.

Use cases:
- Update firewall configuration.
- Update configuration of anything that needs to bind to the correct
  port.

port_forwarding.sh

@@ -130,6 +130,8 @@ Trying to bind the port... "
 # We will repeat this request every 15 minutes, in order to keep the port
 # alive. The servers have no mechanism to track your activity, so they
 # will just delete the port forwarding if you don't send keepalives.
+
+old_port=0
 while true; do
   bind_port_response="$(curl -Gs -m 5 \
     --connect-to "$PF_HOSTNAME::$PF_GATEWAY:" \
@@ -146,6 +148,15 @@ while true; do
       echo -e "${RED}The API did not return OK when trying to bind port... Exiting."
       exit 1
     fi
+
+    if [[ "${PF_POST_COMMAND}" ]] && [[ "${port}" != "${old_port}" ]]; then
+      echo -e "${GREEN}Note: Port has changed from ${old_port} to ${port}. Running PF_POST_COMMAND.${NC}"
+      if ! "${PF_POST_COMMAND}" "${port}" "${old_port}"; then
+        echo -e "${RED}Warning: "${PF_POST_COMMAND}" returned non-zero.${NC}"
+      fi
+      old_port="${port}"
+    fi
+
     echo -e Forwarded port'\t'${GREEN}$port${NC}
     echo -e Refreshed on'\t'${GREEN}$(date)${NC}
     echo -e Expires on'\t'${RED}$(date --date="$expires_at")${NC}