core: Add NMPlatformSource to nm-types.h

It's going to be useful outside nm-platform, to weight MTU options from various
sources.
core: Track origin of MTU

Only override MTU if it came from a source of higher priority or is of equal
priority but of lower value.
rdisc,device: Set MTU if an appropriate option is present in a RA

https://bugzilla.gnome.org/show_bug.cgi?id=738104

Reported-by: Charles R. Anderson <cra@wpi.edu>