Log an arbitrary message.
This is used by all other logging functions. Override
it if you have specific logging wishes.
The first argument, FORMAT, is a format string for the
message to be logged. If the format string contains
any % escapes requiring parameters, they should be
specified as subsequent arguments (it's just like
printf!).
The client ip and current date/time are prefixed to
every message.
Unicode control characters are replaced with escaped hex
before writing the output to stderr.