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.