wgengine,cmd/tailscaled,feature/bird: move BIRD integration to ./feature

The BIRD (BGP) integration previously lived half in cmd/tailscaled
(which created a chirp client via a build-tag-gated file on some
platforms) and half in wgengine (which carried the client in its
Config and toggled the "tailscale" protocol as the node gained or
lost primary subnet router duty).

Move it all to a new feature/bird package, installed on the engine
via the new wgengine.HookNewBird hook, like other feature/* packages.
wgengine.Config.BIRDClient (and the wgengine.BIRDClient interface)
are replaced by a BIRDSocket path from which the engine constructs
the feature's Bird handle at startup. The subnet router overlap
detection and protocol toggling move into feature/bird, preserving
the previous ordering: state is recomputed before Reconfig's
ErrNoChanges early return and applied after the router is configured.

tailscaled keeps BIRD support by default on the platforms that
previously had it (linux, darwin, freebsd, openbsd) via
feature/condregister.

Also, add an integration test, as this feature lacked much test
coverage previously.

Updates #12614

Change-Id: I7866a50779e454c87933b358735f7dcd9e2b126f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/tsnet_test.go

@@ -3409,6 +3409,8 @@ func TestDeps(t *testing.T) {
 		BadDeps: map[string]string{
 			"golang.org/x/crypto/ssh":                       "tsnet should not depend on SSH",
 			"golang.org/x/crypto/ssh/internal/bcrypt_pbkdf": "tsnet should not depend on SSH",
+			"tailscale.com/chirp":                           "tsnet should not depend on BIRD integration",
+			"tailscale.com/feature/bird":                    "tsnet should not depend on BIRD integration",
 			"tailscale.com/feature/clientupdate":            "tsnet should not depend on feature/clientupdate",
 			"tailscale.com/feature/remoteconfig":            "tsnet should not depend on feature/remoteconfig",
 			"tailscale.com/feature/syspolicy":               "tsnet should not depend on syspolicy",