Change the socket search to check both the remote and local fields of the
socket match the socket we want to find.

Sockets are identified by the 4-uple

	local_addr, local_port, remote_addr, remote_port

Two socket can differ by a single one of this four elements.
It is possible for the same local_port to be used by two different sockets,
as long as they are connecting to different remote addresses (or remote
ports).

An example of this bug in action can be seen at:

https://github.com/golang/vscode-go/runs/3141270564?check_suite_focus=true

There the server starts listening on 127.0.0.1:46011 and rejects a valid
client connection by finding the following socket:

60: 0100007F:DD82 0100007F:962D 06 00000000:00000000 03:00000133 00000000     0        0 0 3 0000000000000000

the local address of this socket is 0100007F:DD82 (127.0.0.1:56706), and the
remote address is 0100007F:962D (127.0.0.1:38445).

The reported error is:

	closing connection from different user (127.0.0.1:56706): connections to localhost are only accepted from the same UNIX user for security reasons

note how the local port does match the socket line (56706) but the remote
port is wrong (38445 instead of 46011).

Note also that the state of this socket is 06, or TIME_WAIT, which would be
impossible if this was the right socket, since the right socket would still
be open.

Fixes https://github.com/golang/vscode-go/issues/1555

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.

This prevents other users on the same machine (e.g. a production server) from
using dlv under the credentials of another user, which poses a security issue.