* Add garbage collector command to launcher
The seafile garbage collector can be started via the 'gc' command option of the launcher.
This will stop the seafile-server inside the container, then run the 'seaf-gc.sh' script and redirect its output to '/var/log/gc.log'.
Afterwards the whole container will be restarted.
* Move garbage collector execution logic to scripts/gc.sh
The file gc.sh stops the seafile server, starts the garbage collector and restarts the seafile server when the cleanup is finished.
The output of the 'seaf-gc.sh' script is piped to the file '/var/log/gc.log' in append mode.
* Add 'scripts/gc.sh' to watch_controller function in 'scripts/start.py'
The 'start.py' script monitors the seafile-server in a 'watch_controller' function and terminates the container when the server crashed.
However, during a garbage collector cleanup the server needs to be shut down and therefore it is necessary that the 'watch_controller' function only terminates the container if the server is offline and no cleanup is in progress.
* Add the gc command info to README
* Preseve the exit code of seaf-gc.sh
* Fix the perm of scripts/gc.sh
