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>
core: Use tokenized identifiers when constructing an address

We trigger a new solicitation upon seeing the new token. Kernel triggers one
too, but that one is of no use to us, since the advertisement might arrive sooner
than we learn about the token change.
build: Check for tokenized identifier support in libnl-route-3
platform: Refresh link cache when IPv6 tokenized identifier changes

Refactor the object comparison logic into a separate function. One from libnl
is not good enough (see comment).

Also, kernel currently won't send notifications on token change, a patch is
needed.
platform: Add support for IPv6 tokenized identifiers on Linux
Merge remote-tracking branches 'lr/lr-unmanaged-slaves', 'lr/lr-bluez5-dun', 'lr/lr-ipv6-tokens' and 'remotes/lr/lr-rdisc-mtu' into integration