Support being given a directory for the output path (-p flag)

This should fix #656 and maybe a few other things.
rpm: Add a option to insert raw Require statements

This can be useful to have requirements for different stages.

Examples:

    --rpm-add-require 'Requires(post): /usr/sbin/alternatives'
    --rpm-add-require 'Requires(preun): /usr/sbin/alternatives'