SSLServer: Poll for POST data correctly

This updates the do_POST() code to cleave more closely to the code in
SimpleXMLRPCHandler.do_POST (plus SSL magic, of course).  In doing so,
it eliminates a select() call whose timeouts were not handled
properly, and which thus left stray idle processes around after
incomplete XML-RPC calls.
POSIX: fixed unit tests for 1f0fa73