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