rename configure.in -> configure.ac

Latest autotools warn now if the file is named configure.in

--HG--
rename : configure.in => configure.ac
Define _GNU_SOURCE for vasprintf
send out our kexinit packet before blocking to read the SSH version string
Don't bother waiting for a ssh-connection service reply - the server
will disconnect if it wasn't accepted
Try using writev() for writing packets out to tcp
Fix incorrect logic for USE_VFORK and calling arg_setup()
Send an auth packet straight away, save another roundtrip

This needs a bit of testing to make sure it doesn't have side-effects.
Move the more verbose TRACE() statements into TRACE2()
Try password before interactive - bit of a hack
Run the cleanup handler also when we close due to TCP connection being closed
Be a bit more careful about when we want to use CLI_AUTH_IMMEDIATE

Only use it if we have pubkeys to try, or we have $DROPBEAR_PASSWORD set