fatal usually appends the error string. Add 'fatalx' that doesn't. Fix callers and move the prototypes to log.h

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

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.

matches found with % grep -R '=[ ]*{' . | fgrep -v const

Include gmid.h as first header in every file, as it then includes config.h (that defines _GNU_SOURCE for instance). Fix also a warning about unsigned vs signed const char pointers in openssl.

it also does an overflow check on multiplication, other than being more readable.