Make “cvt” a shared library and move the “cvt” utility to its own repo

As issues #707 (moved) and #868 show, cvt is in need of a bit of TLC.

But we could go a few steps further in making it a project of its own, turning it into a shared library that other projects could use (e.g. Xwayland currently duplicates some code from cvt for its -fake- xrandr, mutter currently uses cvt` at build time but could as well use it at runtime, etc.).

CC: @ajax, @jadahl

added RFE label

I plan to give this idea try, we'll see how far it goes.

I have created https://gitlab.freedesktop.org/ofourdan/libxcvt for that purpose.

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