Add option to use wl_display_add_socket_auto

Summary:
If automaticSocketNaming is enabled, it will use wl_display_add_socket_auto
to allocate the next free socket. The resulting name can be retrieved using
socketName after a successful start afterwards.

Test Plan: Ran the new autotest, passes. kwin_wayland still uses the old behaviour.

Reviewers: #kwin, #plasma, romangg

Reviewed By: #kwin, #plasma, romangg

Subscribers: davidedmundson, zzag, romangg, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D17122