[packagekit] Adding a AllowInterrupt signal

[Richard Hughes]

Guys:

I've just added new API, quoting from helpers/README:

If you have a multipart transaction that can be aborted in one phase but not
another then the AllowInterrupt signal can be sent.
This allows for example the yum download to be cancelled, but not the install
transaction. By cancelling a job all subtransactions are killed.
By default actions cannot be cancelled unless enabled in the compiled backend.
Use "allow-interrupt<tab>true" to enable cancellation and
"allow-interrupt<tab>false" to disable it. This can be done for any job type.

So, for the yum backend we want to emit to stderr
"allow-interrupt<tab>true" when downloading and then emit
"allow-interrupt<tab>false" when starting the rpm transaction. This
lets the user actually stop the job with a cancel button just like you
can with control-c on the command line.

Comments welcome. Commits more welcome :-)

Richard.