update valid license values

* change `:free` (closed source) to `:gratis` for clarity
* add `:freemium` (*eg* Alfred)
* add `:lppl` (LaTeX)
* change `:ubuntu` to `:ubuntu_font`

~~**WIP** pending discussion here and/or in IRC.   In particular, I think `:free` is a poor name, and would lead to confusion.~~ After some discussion on IRC, these changes seem OK — good enough to move forward.

There will always be rough points.  `license` is intended for search/filtering, not annotation.  The most accurate information is always the distribution's home page.

In IRC, we also discussed whether all licenses under `:closed` should be eliminated in favor of just `:closed`.  For the record, I am open to that, if licensing distinctions inspire a too much argument and absorb non-trivial maintainer time.