add gc command to cvmfs_server help text
add gc command to cvmfs_server help text

Gangbiao/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,trshaffer/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,djw8605/cvmfs,djw8605/cvmfs,trshaffer/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,djw8605/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs