libxcmd: merge command() and iterate_command()

libxcmd: merge command() and iterate_command()

Simplify the command loop further by merging the command loop
iteration checks with the command execution function. This removes
all visibility of command iteration from the main command execution
loop, and enables us to factor and clean up the command loop
processing neatly.

[ sandeen: align process_input() args ]

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