[perl #95034] Make perldoc <url> use a reasonabe name

[perl #95034] Make perldoc <url> use a reasonabe name

This patch makes perldoc with a URL use a somewhat reasonable name
based on the basename of the URL, instead of displaying PALLEGCSYO or
BJ1KKH1675 in the pod header (based on the tmp file name).

It treats all URLs the same (changing perldoc.pod to PERLDOC), instead
of treating .pm’s specially.  Maybe this could be improved later.