spksrc: Add Firewall/port-forwarding support

Proposal for adding firewall support into spksrc.
See "Install Package Related Ports Information into DSM" of Synology Developers Guide for background and more info on .sc-file contents.

Usage:
Makefile
Add `FWPORTS = src/${SPK_NAME}.sc`

Installer.sh
Add var `FWPORTS="/var/packages/${PACKAGE}/scripts/${PACKAGE}.sc"`
Add var `SERVICETOOL="/usr/syno/bin/servicetool"`
In postinst (), add:
`# Add port-forwarding config
${SERVICETOOL} --install-configure-file --package ${FWPORTS}`
In preuninst (), add:
`# Remove port-forwarding config
if [ "${SYNOPKG_PKG_STATUS}" == "UNINSTALL" ]; then
    ${SERVICETOOL} --remove-configure-file --package ${PACKAGE}.sc
fi'`