Update to track btsync release, /files tidyup & systemd unit updates

Update 1.3.x branch ebuild to latest btsync version 1.3.106
Conform /files folder structure to Gentoo ebuild packaging guidelines
Add systemd pre-exec setup script to work with all systemd units - creating an initial "stock" .conf file
(restrict to localhost webui access - listening on port based on UID, use hostname as device name, create a  stock "secure" webui password)
Correct systemd user unit install (currently using a "workaround")