Fix various problems
    
    - core: fix reading physical port id for sysfs
    - libnm-glib: export get_gateway() and get_searches() functions
    - libnm-glib: new functions for bindings to get nameservers
    - libnm-glib: chain up the parent constructed() of NMRemoteConnection
    - core: exit cleanly if D-Bus cannot be initialized (rh #1057738)
    - dhcp: don't add an IPv6 address if one wasn't given (rh #1048046)
    - core: Add host route for DHCP4 server if outside assigned subnet (bgo #721767) (rh #983325)
    - vpn: handle missing tunnel interface for IP-based VPNs (bgo #721724) (rh #1030068) (rh #865883)
    - core: only log about IPv6 Commit the first time (rh #1044757)