Fix SSH2_MSG_CHANNEL_EXTENDED_DATA in logparse.

Fix SSH2_MSG_CHANNEL_EXTENDED_DATA in logparse.

It looks as if it's never worked at all: it had a spurious second
printf, it completely forgot to allow for the uint32 type code that
SSH2_MSG_CHANNEL_DATA doesn't have, it accessed the channel state's
sequence number fields in a way that made no sense and didn't match
the rest of the program, *and* it misinvoked the file opening API. I
must have never had an occasion to test it.

[originally from svn r10037]