ssh/tailssh: fix SSH on busybox systems

This involved the following:

1. Pass the su command path as first of args in call to unix.Exec to make sure that busybox sees the correct program name.
   Busybox is a single executable userspace that implements various core userspace commands in a single binary. You'll
   see it used via symlinking, so that for example /bin/su symlinks to /bin/busybox. Busybox knows that you're trying
   to execute /bin/su because argv[0] is '/bin/su'. When we called unix.Exec, we weren't including the program name for
   argv[0], which caused busybox to fail with 'applet not found', meaning that it didn't know which command it was
   supposed to run.
2. Tell su to whitelist the SSH_AUTH_SOCK environment variable in order to support ssh agent forwarding.
3. Run integration tests on alpine, which uses busybox.
4. Increment CurrentCapabilityVersion to allow turning on SSH V2 behavior from control.

Fixes #12849

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ssh/tailssh/tailssh_integration_test.go

@@ -349,7 +349,7 @@ func TestSSHAgentForwarding(t *testing.T) {
 
 	// Run tailscale SSH server and connect to it
 	username := "testuser"
-	tailscaleAddr := testServer(t, username, false) // TODO: make this false to use V2 behavior
+	tailscaleAddr := testServer(t, username, false)
 	tcl, err := ssh.Dial("tcp", tailscaleAddr, &ssh.ClientConfig{
 		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
 	})
@@ -387,7 +387,7 @@ func TestSSHAgentForwarding(t *testing.T) {
 
 	o, err := s.CombinedOutput(fmt.Sprintf(`ssh -T -o StrictHostKeyChecking=no -p %s upstreamuser@%s "true"`, upstreamPort, upstreamHost))
 	if err != nil {
-		t.Fatalf("unable to call true command: %s\n%s", err, o)
+		t.Fatalf("unable to call true command: %s\n%s\n-------------------------", err, o)
 	}
 }