Move certain settings to system-wide profile.

This allows you to customize your own profile without breaking
critical features. When this is stored in user-level settings it's
really easy to override the config in ways that break xorg.

The most important thing is moving the ETNA_MESA_DEBUG=nir setting. I
suspect the unicode_start call is also suitable for the system-wide
profile, but I'm not so sure about the GTK2_RC_FILES; maybe that
should remain in per-user settings? But if a user wants to override
it, their changes will override the system-wide ones anyway.