basic/log: expose log_dispatch

basic/log: expose log_dispatch

This is useful when we want to avoid printf formatting on the message.
It's nicer than using log_struct with "%s" as the format, because printf
is slow and with a large message (like from a backtrace) this would require
extra unnecessary memory.

I'm not exposing all the fields in the wrapper: only level and errno.
Those are the most likely to be useful.