Implemented |##device-port-wait-for-output!| in lib/_io.scm as a complement to |##device-port-wait-for-input!| defined in the same file.

Implemented |##device-port-wait-for-output!| in lib/_io.scm , as a complement to |##device-port-wait-for-input!| defined in the same file. If not else, the combination of these two procedures make a working concept of using Gambit as IO multiplexer for a program that does the actual socket reads and writes in C. This is a usecase that has been proven reliable and is really valuable. Suggesting it for inclusion in Gambit as implementing it in user code relies on an unelegant reverse-engineering hack of the port object structure that may break for future Gambit versions. This commit is public domain, I claim no copyright on it.