dap: add sameuser check (#2494)

On linux, delve RPC server allows only connections from the same user
if --only-same-user is set (true, by default). Do the same for DAP
server.

Moved the sameuser check logic to service/internal/sameuser.
Considered importing service/rpccommon from the dap server,
but when we eventually migrate to multiplex rpc and dap from one
port, I am afraid that can cause cyclic imports.

service/internal/sameuser/doc.go

@@ -0,0 +1,3 @@
+// Package sameuser provides utilities for checking users of a local connection.
+// Only works in Linux.
+package sameuser