Log::Dispatch construction: do not set a callback if log_pid == 0

The callbacks in Log::Dispatch are a hot path as they are called on every log. This is an optimisation of the callbacks set by Dispatchouli to not check if log_pid is set on every call.
The Log::Dispatchouli construction is modified to set optimized
callbacks:
- if log_pid == 0, do not set a callback
- if log_pid == 1, set a optimized callback that just adds the "[$$] "
  prefix, without condition.