[PATCH] network configuration fixes

The patchset contains fixes for network configuration:

Move from modifying ifcfg files to using NM dbus api (settings objects) where possible. [1/8, 2/8, 8/8] Stop writing /etc/sysconfig/network.[3/8] Fix disabling of ipv6. [4/8, 5/8] Don't crash on invalid network --device specification. [6/8] Make modifying ifcfg files more robust (don't crash on missing file, eg for WWAN devices). [7/8, 1/8]