ssh: Fallback to client or server selected version for parsing

In half-duplex setups (or when client/server coalesce the SSH version
line with the KEX packet, get_version() would return UNK as version,
causing a protocol violation. Make this slightly more robust by using
and setting the version which either side had set to continue parsing.

For the special case of SSH-1.99, select SSH-2.0. We could try to peak
into the payload following the packet length field and check for
a KEX_INIT type byte to select SSH2 as a heuristic, but not sure how
to accomplish this.

Slight regression fix for 3769ed6c66
which started to require visibility for client and server version
rather than just the client's version.

testing/btest/scripts/base/protocols/ssh/half-duplex-client.zeek

@@ -5,4 +5,4 @@
 # @TEST-EXEC: btest-diff analyzer.log
 # @TEST-EXEC: btest-diff ssh.log
 # @TEST-EXEC: btest-diff conn.log
-# @TEST-EXEC: btest-diff .stdout
+# @TEST-EXEC: btest-diff weird.log

testing/btest/scripts/base/protocols/ssh/half-duplex-server.zeek

@@ -5,4 +5,4 @@
 # @TEST-EXEC: btest-diff analyzer.log
 # @TEST-EXEC: btest-diff ssh.log
 # @TEST-EXEC: btest-diff conn.log
-# @TEST-EXEC: btest-diff .stdout
+# @TEST-EXEC: btest-diff weird.log