Changed print_file_list to resolve anon node names.

Without resolving anonymous node names, this function fails to
print the file list for a large launch file tree because somewhere
in the tree there are duplicate anonymous node names.  The error
message is:

roslaunch file contains multiple nodes named [/$(anon foo)].
Please check all <node> 'name' attributes to make sure they are unique.
Also check that $(anon id) use different ids.

I don't see what the node name anonymous-ness has to do with listing
the launch files, so it seems like a safe and useful change.