version: use OSS repo's version when building.

When building with redo, also include the git commit hash
from the proprietary repo, so that we have a precise commit
that identifies all build info (including Go toolchain version).

Add a top-level build script demonstrating to downstream distros
how to burn the right information into builds.

Adjust `tailscale version` to print commit hashes when available.

Fixes #841.

Signed-off-by: David Anderson <danderson@tailscale.com>

derp/derp_server.go

@@ -1291,7 +1291,7 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("multiforwarder_deleted", &s.multiForwarderDeleted)
 	m.Set("packet_forwarder_delete_other_value", &s.removePktForwardOther)
 	var expvarVersion expvar.String
-	expvarVersion.Set(version.LONG)
+	expvarVersion.Set(version.Long)
 	m.Set("version", &expvarVersion)
 	return m
 }