[usocket-devel] more convenient :nodeay option - :if-supported

[Anton Vodonosov]

Hello.

Today socket-connect option :nodelay if specified accepts T or NIL. And whatever value is passed,
socket-connect signals the UNSUPPORTED exception on implementations where it is not
possible to control the socket no-delay property.

This dooms any usocket-using code which wishes to use :nodelay to be broken on
implementations where it is not implemented (e.g. CLISP, CMUCL, ...).

I think in most cases users would prefer "no delay if possible, otherwise just work as you can"
behavior. For example drakma. On the implementations where it is possible, it will have 
maximum performance by specifying no-delay. But on other implementations it is desirable
to work on usual socket.

To support this in backward-compatible fashion I propose to introduce another value
for the :nodelay option. :nodelay :if-supported. The meaning is obvious.

Please see the patch attached. It is tested (by changing drakma to use :nodelay :if-supported)
on all the implementations available to me: Allegro, CCL, CLISP, SBCL. 

Can't test on ECL and ABCL because of other obstacles (the last public release of ECL
can't comile babel, and ABCL can't load cffi).

Best regards,
- Anton
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