hammer - Add ssh-remote directive

hammer - Add ssh-remote directive

* Adds a feature that allows you to set up a command="..." prefix for a
  ssh key in your ~/.ssh/authorized_keys file that only runs the hammer
  utility and only with specific commands and filesystem paths.

  For example:

  command="/sbin/hammer ssh-remote mirror-read,mirror-write /path/" ssh-rsa...

  Currently requires a trailing '/' if you want to restrict the path to
  a subdirectory.  Multiple commands can be listed but the filesystem path
  restriction is only currently tested for mirror-read and mirror-write.

* This allows ssh to be used for mirroring without having to give shell
  access to the remote.