Fix non-inclusive terms

[rkratky]

This changes:

• 'master' to 'main' -- that's in reference to the Ubuntu archive, so even though it might not be called that, I expect it should be self-explanatory and not a problematic change (even if only in the docs for the time being).
• 'blacklist' to 'blocklist' -- in germinate terminology. This is more complicated because germinate, the tool itself, is full of the word. So, to get things moving, I submitted a bug against the tool (Bug #2114677), as well as a first naive attempt at fixing it in the tool (MP #487117). Let's see how that goes...

[s-makin]

lgtm :) thanks for clarifying