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/sameuser.go

@@ -0,0 +1,9 @@
+//+build !linux
+
+package sameuser
+
+import "net"
+
+func CanAccept(_, _ net.Addr) bool {
+	return true
+}