Make PeerDrive startable as user daemon

By using erldaemon it's now possible to start PeerDrive per user
session. A configuration file is not strictly necessary because
there are common default settings for the system store and the
front ends.
Allow multiple instances of the same interface
Dynamically configure client interface port

The TCP port for the client interface is bound dynamically by default.
Additionally the client has to provide a cookie when connecting to the
server. Both information is stored in a server.info which the client
reads upon startup to find the server.

This ensures that multiple users can run their PeerDrive daemons safely
side by side on a shared server. A globally running daemon is still
supported, though.
Start PeerDrive daemon per-user by default

The PeerDrive daemon is now started by an autostart .desktop file for
each user on login. It is still possible to start PeerDrive via init as
system daemon by adjusting /etc/default/peerdrive.

Unfortunately the daemon will still live after the logout of the user
because the integration with XSMP/ConsoleKit is lacking. In the long run
systemd user sessions should handle it, just like launchd does already
on OSX.
netwatch: add simple start/stop eunit test