Fix ebuilds. Lower systemd service priority. Update stable branch.

 ebuild variable references fixed
 ebuild's cleaned up and slimmed down
 stable branch of btsync updated from 1.3.94 to 1.3.105
 btsync binary install path corrected to Gentoo standard: /opt/
 systemd service unit CPU & I/O priority lowered to idle
 systemd service units reference corrected btsync binary path
 OpenRC/init.d files tidied up and reference corrected binary path
 Manifest file rebuilt