add --channel option to find out RPMs in channels; narrowing search space with software channels specified, in rpmkit.identrpm
arguments list must be composed to a string passed to '-A' option in rpmkit.swapi._call
handle cases if some args are not str in rpmkit.swapi._call
add --nprocs option to limit parallel processes in rpmkit.identrpm
make rpmkit.identrpm.list_rpms_in_channel a little more verbose