leftover from when README.md was generated by gmid.1

we still have an hardcoded list, but this implements the API needed to modify the mappings.

err/warn is not available on some systems (unfortunately!) and in any case don't play well with our daemon mode (that closes std{in,out,err}). Use our fatal that is daemon-aware.

when we switched from one process to two, I introduced a small optimisation: empty string are not send, so we receive NULL. Constructing requri we need to make sure that relpath is not null.

we don't fork anymore in that process, so the flag is extra.

cgi.c wasn't really needed; it better to group all the server related functions together, cgi or not. Now gmid.c contains only startup and utility code.