btrfs: use exit, not return
provide infrastructure to add new scripts

-call pre-backup.sh, post-backup.sh, pre-share.sh, post-share.sh before
and after each backup / shareDump
-enable hooks in backuppc-tools.conf
-define functions in topic-specific files (e.g. snapshot.sh)
  * function some_func is called if conf_some_func is set to 1
  * all args passed to the main script are passed to all functions