Allow byte-swapped clients in XWayland via some configuration

The X Server will disallow clients with differing endianess in the future.

Historically, the X client has sent its own endianess in the connection request (l or B) with the server then - if required - swapping every request, reply, event from the client's announced endianess to the server-native endianess. This swapping code is hand-written, little tested and very much at the mercy for malicious clients. Quite a number of security issues have been found with that code over the years. Hence the proposed change to simply disallow connections from clients that use different endianess. This is a defaults change.

Differing endianess is a niche use-case but still present, see this fedora-devel thread.

To allow clients with different endianess to connect, the proposed X server PR adds an AllowSwappedClients xorg.conf option for Xorg and a commandline option +byteswappedclients for all X server implementations, including Xwayland. In Xwayland it becomes wlroot's job to pass that flag through to Xwayland - which means there has to be a configurable option somewhere, we definitely don't want this enabled on 99.9% of all users.

Note that at the time of writing, the xserver PR is not yet merged and some changes to option naming may happen.