Add the identity command and serve it on an opt-in HTTP info API

Build slackhq/nebula#1821 ("who is this vpn address?" over a local
HTTP+JSON socket) on the command registry, so the lookup is one command
on every surface and the socket is one more local API.

identity
- `identity [-address <vpn addr>[:port]]`: no address reports this node;
  with one, this node or the peer behind an established tunnel
- Certificate fields only: name, vpn addresses, networks, unsafe
  networks, groups, fingerprint, issuer, validity, cert version; a peer's
  fingerprint is the one nebula computed when it verified the cert
- Port accepted and stripped, 4in6 unmapped, so a RemoteAddr passes
  straight through
- No tunnel is a commands.NotFound; an empty -address is refused
- Lighthouse TXT lookup shares the owner and peer lookups, keeps answering
  with the initiating certificate, and no longer panics without a default cert
- Served on ssh and nebula ctl too

Info API
- Off until `local_api.info.enabled` with `socket` and optional quoted
  octal `socket_mode` (default "0600"); nothing derived
- Serves commands declaring Command.Identity plus help; others 403 naming
  their surfaces. TestInfoAPISurface pins the set to identity
- GET /v1 is the schema; GET /v1/<cmd>?<flag>=<value> runs one; body is
  the -json rendering via commands.Render; commands.PresentationFormat
  reads json/pretty from the query the way a line is read
- 400 commands.ParamError, 403 commands.WithheldError (what Resolve now
  returns for a withheld name, so help and dispatch type-check it too),
  404 NotFound or unknown, 500 Run error, refusal with no reason, or
  recovered panic with no detail; no redirects; every miss is JSON
- listen takes a mode: bind under umask 0077 then chmod; "0660" plus a
  searchable dir opens it to a group
- The chmod, at bind and on a mode-only reload, goes through chmodSocket,
  which never follows a link: a user who can write the socket's directory
  could otherwise swap the node for a link and have a root daemon put
  0660 or 0666 on any file. Only a socket we own is chmoded. On linux the
  node is held open O_PATH|O_NOFOLLOW, checked by fstat, and chmoded
  through /proc/self/fd, so the check and the chmod land on one inode;
  fchmodat2 is the fallback without /proc. On darwin and the BSDs it is
  an Lstat check and fchmodat(AT_SYMLINK_NOFOLLOW). clearStale Lstats
  too, so a dangling link is refused rather than followed by bind on
  darwin
- One endpoint type owns bind under mutex, bind-before-release move,
  context watch and stop/unlink; each server supplies what it serves
- Reloadable through the same reconcile shape as the control socket:
  mode applied in place, unparsable section keeps what runs, control
  socket path refused by name
- Wire returns Endpoints{Control, Info} with one Stop
- Windows unsupported for now (a pipe has no mode); warned once

Tests: identity lookups and shapes; API filter, status codes, panic
recovery, socket mode, rebind, cancellation; config parsing incl. bare
0660; Wire transitions; e2e of two nebulas with the info API resolving
the peer by address and port, through shared test.HTTPDo/HTTPGet helpers.
Goldens gain identity.

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

test/helpers.go

@@ -1,7 +1,11 @@
 package test
 
 import (
+	"context"
 	"flag"
+	"io"
+	"net"
+	"net/http"
 	"os"
 	"path/filepath"
 	"testing"
@@ -39,3 +43,34 @@ func Golden(t *testing.T, path, got string) {
 	require.NoError(t, err, "missing golden file, run with -update")
 	assert.Equal(t, string(want), got)
 }
+
+// HTTPDo sends one request to the HTTP server on the unix socket at socket and returns
+// the status, body and headers of its answer. The host in the URL is a placeholder:
+// every connection goes to the socket.
+func HTTPDo(t *testing.T, socket, method, target string) (int, string, http.Header) {
+	t.Helper()
+
+	client := &http.Client{Transport: &http.Transport{
+		DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
+			return (&net.Dialer{}).DialContext(ctx, "unix", socket)
+		},
+	}}
+	t.Cleanup(client.CloseIdleConnections)
+
+	req, err := http.NewRequest(method, "http://nebula"+target, nil)
+	require.NoError(t, err)
+	res, err := client.Do(req)
+	require.NoError(t, err)
+	defer res.Body.Close()
+
+	body, err := io.ReadAll(res.Body)
+	require.NoError(t, err)
+	return res.StatusCode, string(body), res.Header
+}
+
+// HTTPGet is HTTPDo for a GET whose headers the test does not read.
+func HTTPGet(t *testing.T, socket, target string) (int, string) {
+	t.Helper()
+	status, body, _ := HTTPDo(t, socket, http.MethodGet, target)
+	return status, body
+}