Fix user-facing typos (mainly in descriptions)

I'm no fan of wanton clean-ups and don't mind the occasional mistake, but one should probably avoid things like "ouput" and "developement" in package descriptions just for the sake of searchability.

Hence a light New Year's clean-up patch. Bring user-visible strings closer to the same high quality as the code. Increase user confidence. Professionalism. Become more attractive. Exercise.

I didn't mess with the often awkward grammar, or spelling errors in comments, code*, and other "user-invisible" text. Nor did I touch any auto-generated files that were clearly marked as such.

[* Apart from "mlmmj-recieve", which is just wrong.]