[Bug #1473048]

[Bug #1473048]
SimpleXMLRPCServer and DocXMLRPCServer don't look at
the path of the HTTP request at all; you can POST or
GET from / or /RPC2 or /blahblahblah with the same results.
Security scanners that look for /cgi-bin/phf will therefore report
lots of vulnerabilities.

Fix: add a .rpc_paths attribute to the SimpleXMLRPCServer class,
and report a 404 error if the path isn't on the allowed list.

Possibly-controversial aspect of this change: the default makes only
'/' and '/RPC2' legal.  Maybe this will break people's applications
(though I doubt it).  We could just set the default to an empty tuple,
which would exactly match the current behaviour.