pre-lib9p servers: fix incorrect Tversion handling

pre-lib9p servers: fix incorrect Tversion handling

version(5) says:

	If the server does not understand the client's version
	string, it should respond with an Rversion message (not
	Rerror) with the version string the 7 characters
	``unknown''.

Pre-lib9p file servers -- all except cwfs(4) -- do return Rerror.

lib9p(2) follows the above spec, although ignoring the next part
concerning comparison after period-stripping.  It assumes an
Fcall.version starting with "9P" is correctly formed and returns
the only supported version of the protocol, which seems alright.

This patch brings pre-lib9p servers in accordance with the spec.