Source configuration files in /etc/mkinitcpio.conf.d/

1. Comment out ALL_config by default in preset files: use the default configuration file (e.g. /etc/mkinitcpio.conf, and only use {PRESET}_config or ALL_config if it is specifically desired (i.e. not commented out in presets). I think this is a better default and should be kept whether or not we keep the rest of this MR.
2. Source additional configuration files, but only when using the default /etc/mkinitcpio.conf (i.e., not when a configuration file is specified in a preset).