Syncthing should not try to start a browser when invoked by systemd.
Furthermore we do not need any timestamps in the journal as systemd
already handles this for us.
- Removed environment file to keep the service file minimal.
"systemctl edit syncthing.service" does the job if somebody wants
to customize the service.
- Changed "cmdline.target" to "default.target" as "cmdline.target"
does not exist in systemd.special:
http://www.freedesktop.org/software/systemd/man/systemd.special.html
- Added a missing "After=network.target".
- Added a documentation hint, thx @jaystrictor
