`kmscon.conf` should support layered configuration with drop-in files

[Conan-Kudo]

In general, Linux distributions, Linux admins, and Linux users prefer to see layered configuration for stuff like this (it's called "hermetic /usr" with the Configuration Files Specification by the systemd folks, "Stateless Linux" by Intel and Solus OS, etc.).

In particular, for the Fedora case, I expect that we're going to have some Fedora variants who want the mouse support enabled by default and others who don't care, and there's currently no way to handle that if the default is to have it switched off.

[kdj0c]

For the mouse support, I think it's a bit too experimental to be enabled by default.
But the goal is that should be enabled by default when it's stable, and works for most people.

Opengl also should be enabled, (if it's not software rendering).

Layered config is still a good idea, so I keep this open.

There is already 2 layers, as the command line arguments are treated like a config file, so it shouldn't be that hard to add more layers.