Reduce code-duplication of socketpair in WaylandServer

Summary:
WaylandServer creates a socketpair for several Wayland connections. So
far this duplicated code quite a bit.

This change introduces one method to perform the socketpair and the
error checking plus creating the server side ClientConnection from the
pair.

Test Plan: Tests still pass

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3576