Add per-user service

With these changes, you can do

    # systemctl start btsync@username

to run the service as user `username`. The service expects to find a config file at `~/.config/btsync/btsync.conf` and you can run this in parallel to the existing btsync.service.

The changes include some changes to btsync.service to allow stopping one of the services without stopping them all. The btsync executable provides a sync.pid in the storage directory, so by pointing systemd to use that, you can run the btsync service for multiple users in parallel, independently of each other.