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.).