add file descriptor detectors to cvmfs_server
add (verbose) warning for busy remount
add kill_cvmfs to cvmfs_suid_helper
add *_lazy_umount to cvmfs_suid_helper
allow `cvmfs_server abort` on busy mountpoint
allow `cvmfs_server publish` on busy mountpoint
allow `cvmfs_server rollback` on busy mountpoint
Merge pull request #434 from reneme/fix-remount_with_ro_fds

FIX: Transaction, Publish, Abort, Rollback with Open File Descriptors