libxcmd: rename args_command to command_iterator

libxcmd: rename args_command to command_iterator

It is not particularly easy to understand the function of the
args_command abstraction. it's actually a command iterator interface
that allows callers to specify the target of the command and iterate
the command multiple times over different targets. Rename and
document the abstraction to make this functionality clear.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>