Previously, nm_setting_diff() (and thus nm_connection_diff()), returned
only properties that are different AND not set to the default value.
However, if the opposite setting 'B' was missing, it would always
include all properties from 'A', even the default ones.

This behaviour was asymetric. Add two new compare flags
@NM_SETTING_COMPARE_FLAG_DIFF_RESULT_WITH_DEFAULT and
@NM_SETTING_COMPARE_FLAG_DIFF_RESULT_NO_DEFAULT to control the
behaviour of whether to include default properties.

Signed-off-by: Thomas Haller <thaller@redhat.com>