Plugin arg handling change now committed

[Ian Booth]

Hi folks

As per a previous email, the way plugin argument handling is done has changed.
The code is now committed to trunk.

The main effects are:
- plugins which expected juju to set JUJU_ENV when -e was used now will need to
do that themselves.
- it is no longer necessary to use "--" to separate juju and plugin args
- plugins receive all args as typed

I think there is currently only one juju-core plugin in use; now is the time to
change it there are users running juju from trunk.