Add a local control socket and nebula ctl (#1975)

Add support for accessing the previously ssh-only debug commands via
`nebula ctl <command>.`

- unix socket (named pipe on windows) for communication
- sockets scoped to the user + an optional group
- in cmd/nebula and cmd/nebula-service, but not nebula.Main

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

go.mod

@@ -6,6 +6,7 @@ require (
 	dario.cat/mergo v1.0.2
 	filippo.io/bigmod v0.1.0
 	github.com/DefinedNet/netlink v1.3.2-dn.1
+	github.com/Microsoft/go-winio v0.6.2
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
 	github.com/flynn/noise v1.1.0
 	github.com/gaissmai/bart v0.29.0