Add plugin calling support

Add plugin calling support

This patch adds a simple plugin system that makes simple problems easy to
solve, and difficult problems, such as SELinux, possible.

When the transaction gets to the point where a collection action should occur,
it expands a macro of the form %__collection_<collection name> to get the path
to a plugin and any additional options. The plugin is dlopen'ed, and the
appropriate function is called in the plugin, with the additional arguments
passed in.

This also adds a --nocollections option to disable performing Collection
actions.