xfstests: clean up and simply check CLI option parsing

xfstests: clean up and simply check CLI option parsing

The options are parsed in a messy loop of option parsing and
actions on secondary arguments. Turn it into something much neater
and easy to understand rather than a mess of temporary variables
and tortured logic...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Phil White <pwhite@sgi.com>
[rjohnston@sgi.com fixed typos in commit message]
Signed-off-by: Rich Johnston <rjohnston@sgi.com>