Major changes in the way socket options work.

* Eliminated GET-SOCKET-OPTION and SET-SOCKET-OPTION
* Replaced GET-SOCKET-OPTION by generic function SOCKET-OPTION
* Replaced SET-SOCKET-OPTION by a SETF expander for SOCKET-OPTION
* Rewrote socket-option-defining macros

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