The goal is reducing log messups (which happen surprisingly often) by
buffering partial lines in mp_log. This is still not 100% reliable, but
better.
The extrabuffers for MSGL_STATUS and MSGL_STATS are not needed anymore,
because a separate mp_log instance can be used if problems really occur.
Also, give up, and replace the snprintf acrobatics with bstr.
mp_log.partial has a quite subtle problem wrt. talloc: talloc parents
can not be used, because there's no lock around the internal talloc
structures associated with mp_log. Thus it has to be freed manually,
even if this happens through a talloc destructor.