A few changes.

* Rename REMOTE-ADDRESS to REMOTE-HOST and LOCAL-ADDRESS to LOCAL-HOST
* Add REMOTE-FILENAME and LOCAL-FILENAME
* Rename SOCKET-FD to SOCKET-OS-FD
* Refactor %SEND-TO
* SEND-TO: rename arg REMOTE-ADDRESS to REMOTE-HOST and add REMOTE-FILENAME
* Add docstrings for external socket generic functions

Signed-off-by: Stelian Ionescu <sionescu@common-lisp.net>