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.
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.