- don't use GSList as intermediate data type to construct
the environment, especially when all we want is to get
a strv array at the end. This reverts the order of elements
compared to previously.
- add and use helper methods _items_add_*() which assert against sensible
input.
- merge IPv4 and IPv6 implementations for creating the environment.
https://github.com/NetworkManager/NetworkManager/pull/112
Originally, the "callouts" directory contained various programs
that NetworkManager would call, for example the dhcp helper.
For a while, it only contains nm-dispatcher. Thus rename the directory
to indicate that it's for dispatcher.
