ZNC: Fix webadmin, allow external config and minor fixes

ZNC: Fix webadmin, allow external config and minor fixes

 * Fix the webadmin module and add the themes

 * Allow providing ZNC an external config (closes #9110)

 * Allow specifying the user and group to run ZNC as (needs su when using
   external config)

 * Allow providing a certificate for incoming SSL connections

 * Reduce buildtime by not rebuilding everything everytime

Signed-off-by: Jonas Gorski <jonas.gorski+openwrt@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@26722 3c298f89-4303-0410-b956-a3cf2f4a3e73