draft-ietf-secsh-transport-24 says that only "SSH-" at the start of a line

marks a version string.  It's a bit vague about the definition of a line,
but I think it's reasonable to assume that they'll end with LF.  Change
do_ssh_init() to ignore "SSH-" anywhere else.  This makes the existing state
machine overkill, so replace it with something a little more readable.

[originally from svn r6138]