RFE: make rpm plugin API public

[pmatilai]

The plugin API should made public, the current situation is missing half the point of plugins by forcing them to live in rpm proper. This was of course always the goal, but things haven't moved anywhere since the plugin system was introduced. There are things that will need addressing first, at least

• package-level file staged unpacking and associated semantics change (Rpm leaves untracked files behind on install failure #967)
• file operations (setmeta etc) should be based around open fd when possible, requires API change