Fix httpretrieve, httpserver, registerhttpcallback, and their unit tests

[aaaaalbert]

httpretrieve is a (less feature-rich) Repy version of Python's urllib. httpserver implements an HTTP server in Repy and abstracts away lots of the protocol details. registerhttpcallback seems to serve a similar purpose.

All of these libraries need to be scrutinized regarding their use of RepyV2 API calls and idioms (like checking for SocketWouldBlockErrors). Furthermore, the unit tests (1) should move into the correct dir, (2) must use the correct module name (seattlelib rather than httpretrieve), and (3) must be ported to RepyV2.

I did an initial cleanup of httpretrieve in aaaaalbert/seattlelib_v2, and created two scripts for testing httpretrieve's HTTP POST in aaaaalbert/repy-doodles, namely test-post and httpserver-with-post.

[aaaaalbert]

Note: The problems in httpretrieve also cause trouble in xmlrpc_client where it is imported, and thus in geoip_client.r2py(see SeattleTestbed/seash#81).