Add -ssh: an opt-in built-in SSH server (public keys only, sessions only)

sshd/run_windows.go

@@ -0,0 +1,12 @@
+package sshd
+
+import "golang.org/x/crypto/ssh"
+
+// Shells need a pseudo-terminal, which Windows has no simple equivalent of here, so the server
+// signs people in but refuses to start a session. main reports this before it starts the server.
+const (
+	ptySupported = false
+	runSupported = false
+)
+
+func run(ssh.Channel, string, *ptyReq, <-chan [2]uint32) int { return 1 }