man: recommend %m over strerror()

man: recommend %m over strerror()

The need to set errno is very very ugly, but at least it is thread-safe and
works correctly. Using strerror() is likely to be wrong, so let's not recommend
that. People who do a lot of logging would provide use some wrapper that sets
errno like we do, so nudge people towards %m.

I tested that all the separate .c files compile cleanly.