Refactored data transfer code.

This is a significant change.
The network code which was previously spread out among
elpher-get-selector, elpher-get-gemini-response and
elpher-get-finger-page has been replaced by a single
elpher-get-host-reponse function.

In addition, we now use make-network-process instead of
open-network-stream, as this affords us greater flexibility
in speicyfing connections.  This allows us to properly
specify ipv4, when necessary.

Connection error reporting has been improved, and connection
state is now explicitly reported.