New hammer utility feature in HEAD: 'hammer cleanup'

[Matthew Dillon]

    I've added a meta-command to the hammer utility.  

    'hammer cleanup' will do the following:

    * Locate all mounted HAMMER filesystems, including PFS's mounted with
      nullfs.

    * Create <fs>/snapshots and <fs>/snapshots/config if they do not exist.

    * Snapshot, prune, and reblock the filesystems based on the config file.
      This command will properly run hammer utility commands with cycle
      files ( <fs>/snapshots/.*.cycle ) and time limits (default 5 minutes)
      per sub-command.

    The config file by default specifies that snapshots, pruning, and
    reblocking should occur daily.  That is, if you run this command
    multiple times it will be a NOP most of the time, until the day turns
    over, then it does its stuff.

    The general idea is to use this command to take all the effort out of
    building your own scripts to do these operations, simply running the
    command without any arguments from cron.

    --

    This command does not yet remove any snapshots.  I intend for it to
    ultimately do that, and possibly even run a 'df' to figure out available
    free space and start removing snapshots when free space gets low.  It
    does not do this yet, however, so you will have to manage removal of
    the snapshots yourself.

							-Matt