Handle socket errors on half-open channels.

Handle socket errors on half-open channels.

Anthony Ho reports that this can occur naturally in some situation
involving Windows 8 + IE 11 and dynamic port forwarding: apparently we
get through the SOCKS negotiation, send our CHANNEL_OPEN, and then
*immediately* suffer a local WSAECONNABORTED error before the server
has sent back its OPEN_CONFIRMATION or OPEN_FAILURE. In this situation
ssh2_channel_check_close was failing to notice that the channel didn't
yet have a valid server id, and sending out a CHANNEL_CLOSE anyway
containing 32 bits of uninitialised nonsense.

We now handle this by turning our half-open CHAN_SOCKDATA_DORMANT into
a half-open CHAN_ZOMBIE, which means in turn that our handler
functions for OPEN_CONFIRMATION and OPEN_FAILURE have to recognise and
handle that case, the former by immediately initiating channel closure
once we _do_ have the channel's server id to do it with.

[originally from svn r10039]