libwebsockets

connect: erase previous unusability when retrying connect

In the case that we try ipv6 that isn't routable, we get a POLLHUP, that
marks the wsi as unusable (for writes, not pending reads), that's what
we want.

But in the case we go around and retry other dns results that are
routable, we have to clear the wsi unusable flag.  Otherwise we will
connect and find that we can't write on the connection...