slirp: check TCP connect completion with SO_ERROR

slirp: check TCP connect completion with SO_ERROR

The nonblocking connect path treated a writable socket as connected
and then used a zero-length send as the failure probe. That is not a
reliable completion check and can mis-handle failed or pending guest
TCP connects.

Use getsockopt(SO_ERROR) when select reports the socket writable. Keep
pending connects pending, and only clear SS_ISFCONNECTING after the
host socket has no pending error.