tool_operhlp: Check for backslashes in get_url_file_name

Extract the filename from the last slash or backslash. Prior to this
change backslashes could be part of the filename.

This change needed for the curl tool built for Cygwin. Refer to the
CYGWIN addendum in advisory 20160127B.

Bug: https://curl.haxx.se/docs/adv_20160127B.html