For the most part, this patch just renames some change-flags, but
doesn't change much about them. The new name should better express
what they are.

A config-change signal can be emitted for different reasons:
when we receive a signal (SIGHUP, SIGUSR1, SIGUSR2) or for internal
reasons like resetting of no-auto-default or setting internal
values.

Depending on the reason, we want to perform different actions.
For example:
 - we reload the configuration from disk on SIGHUP, but not for
   SIGUSR1.
 - For SIGUSR1 and SIGHUP, we want to update-dns, but not for SIGUSR2.

Another part of the change-flags encodes which part of the configuration
actually changed. Often, these parts can only change when re-reading
from disk (e.g. a SIGUSR1 will not change any configuration inside
NMConfig).

Later, we will have more causes, and accordingly more fine-grained
effects of what should be done on reload.