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>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -13,6 +13,7 @@ import (
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -13,6 +13,7 @@ import (
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_linux.go

@@ -13,6 +13,7 @@ import (
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -13,6 +13,7 @@ import (
 	// process and can cache a prior success when a dependency changes.
 	_ "inet.af/netaddr"
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"

tstest/integration/tailscaled_deps_test_windows.go

@@ -16,6 +16,7 @@ import (
 	_ "golang.org/x/sys/windows/svc/mgr"
 	_ "golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	_ "inet.af/netaddr"
+	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"
 	_ "tailscale.com/envknob"