If IPV6CP terminates before IPCP, pppd enters the RUNNING phase and we
start IP configuration without having an IP interface set, which
triggers assertions.
Instead, add a SetIfindex() D-Bus method that gets called by the
plugin when pppd becomes RUNNING. The method sets the IP ifindex of
the device and starts IP configuration.
https://bugzilla.redhat.com/show_bug.cgi?id=1515829
This makes it easier to install the files with proper names.
Also, it makes the makefile rules slightly simpler.
Lastly, the documentation is now generated into docs/api, which makes it
possible to get rid of the awkward relative file names in docbook.
