spksrc: Firewall/port-forwarding support by adding FWPORTS
See "Install Package Related Ports Information into DSM" of Synology Developers Guide

Usage:
Makefile:
- Add FWPORTS = src/${SPK_NAME}.sc
Installer.sh:
- Add FWPORTS="/var/packages/${PACKAGE}/scripts/${PACKAGE}.sc"
- Add SERVICETOOL="/usr/syno/bin/servicetool"
- postinst ():
    # Add port-forwarding config
    ${SERVICETOOL} --install-configure-file --package ${FWPORTS}
- preuninst ():
    # Remove port-forwarding config
    if [ "${SYNOPKG_PKG_STATUS}" == "UNINSTALL" ]; then
        ${SERVICETOOL} --remove-configure-file --package ${PACKAGE}.sc
    fi
Merge pull request #755 from Dr-Bean/br_firewall

spksrc: Add Firewall/port-forwarding support