hammer2 - Cleanup and bugfix pass to hammer2 setcomp paths

hammer2 - Cleanup and bugfix pass to hammer2 setcomp paths

* HAMMER2 will attempt to use LZ4 compression by default now.

* hammer2 setcomp now accepts text labels for the compression type and
  accepts a compression level of 0 to set the default, which is simply
  passed through to the VFS.

* Specify force and recursion flags in hammer2's main options rather than
  after the setcomp command.

* Allow a list of paths to be specified rather than just one.

* The HAMMER2IOC_INODE_SET ioctl now only sets field elements which can
  actually be modified by userland i.e. only the comp_algo field.  It
  used to set everything, including the indirect block table.

* Do not modify the hammer2_inode if the compression method is not being
  changed.

* Change the zero-block testing logic a little, with related code cleanups.