[RFE] log: support %m format specifier regardless if this GNU extension supported by libc

I think this shall only be handled once structured logging (#286)
is resolved since there are some synergetic optimizations, e.g., when
logging to blackbox (turn `%m` into something like "\bm", store `errno`
in the structured payload for a message, only involve `strerror`
equivalent during the interpretation by `qb-blackbox`), but would be
handy to always have this flexibility available regardless of the
libc extra (above POSIX) features.

Last but not least, `qb_perror` definition could then start relying
on that, making it slightly less complex (it's a user facing code)
and more friendly for copycat'ing to one's liking.