Revamp net_pending_errors using toplevel callbacks.

Revamp net_pending_errors using toplevel callbacks.

Again, I've removed the special-purpose ad-hockery from the assorted
front end message loops that dealt with deferred handling of socket
errors, and instead uxnet.c and winnet.c arrange that for themselves
by calling the new general top-level callback mechanism.

git-svn-id: http://svn.tartarus.org/sgt/putty@10023 cda61777-01e9-0310-a592-d414129be87e