tailcfg, ssh/tailssh: optionally support SSH public keys in wire policy

And clean up logging.

Updates #3802

Change-Id: I756dc2d579a16757537142283d791f1d0319f4f0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -63,7 +63,10 @@ func TestMatchRule(t *testing.T) {
 			name: "no-principal",
 			rule: &tailcfg.SSHRule{
 				Action: someAction,
-			},
+				SSHUsers: map[string]string{
+					"*": "ubuntu",
+				}},
+			ci:      &sshConnInfo{},
 			wantErr: errPrincipalMatch,
 		},
 		{