Fix several problems with the test-patches script

Change the test-patches script to fix several pain points:

• It builds packages with the same ABI name as the source package it's run from, so they may not be coinstallable with the current kernel. (related to #1022061 and !657 (closed))
• The linux-headers package it builds is not installable. (#871216)
• It no longer allows disabling debug info, which takes a lot of extra disk space and time to build. (related to #1023773)
• The build may fail due to a patch changing kernel ABI.
• Interrupting it may cause following runs to fail.
• It does a parallel build but under fakeroot, which limits actual concurrency a lot.
• It doesn't work with a clean git checkout.
• It doesn't detect the flavour of backported kernels any more.

Also make some minor changes to gencontrol.py to support this.