cmd/tailscaled/childproc: add be-child registration mechanism

For ssh and maybe windows service babysitter later.

Updates #3802

Change-Id: I7492b98df98971b3fb72d148ba92c2276cca491f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -27,6 +27,7 @@ import (
 	"github.com/creack/pty"
 	"github.com/gliderlabs/ssh"
 	"inet.af/netaddr"
+	"tailscale.com/cmd/tailscaled/childproc"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/net/tsaddr"
@@ -34,6 +35,15 @@ import (
 	"tailscale.com/types/logger"
 )
 
+func init() {
+	childproc.Add("ssh", sshChild)
+}
+
+func sshChild([]string) error {
+	fmt.Println("TODO(maisem): ssh dbus stuff")
+	return nil
+}
+
 // TODO(bradfitz): this is all very temporary as code is temporarily
 // being moved around; it will be restructured and documented in
 // following commits.