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