config: read configuration directory "/usr/lib/NetworkManager/conf.d"

config: read configuration directory "/usr/lib/NetworkManager/conf.d"

This allows packages to install their configuration snippets to
"/usr/", which is a better place for system-provided configuration
files then "/etc".

"/usr/lib/NetworkManager/conf.d/" is read first, so that the values
in /etc have higher priority.

In general, we want to move system-provided configuration away from
/etc, so that a user can do a "factory-reset" by purging /etc.

https://bugzilla.gnome.org/show_bug.cgi?id=738853