and remove the explicit mapping from the documentation and test files.

It's still possible to override this automatic mapping by just adding an entry like application/octet-stream gemini gmi in the `types' block.

if using `types' the internal listing is not used, so show how to add the entries for text/gemini when using /usr/share/misc/mime.types.

add_mime nows allocate dinamically copies of the passed strings, so that we can actually free what we parse from the config file. This matters a lot especially with lengthy `types' block: strings that reach the internal mapping are never free'd, so every manual addition is leaked.