ssh: expose negotiated algorithms

Fixes golang/go#58523
Fixes golang/go#46638

Change-Id: Ic64bd2fdd6e9ec96acac3ed4be842e2fbb15231d
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/538235
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/server.go

@@ -315,6 +315,7 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 
 	// We just did the key change, so the session ID is established.
 	s.sessionID = s.transport.getSessionID()
+	s.algorithms = s.transport.getAlgorithms()
 
 	var packet []byte
 	if packet, err = s.transport.readPacket(); err != nil {