common: LogClient: allow specifying facility for LogClient

common: LogClient: allow specifying facility for LogClient

Instead of allowing only one LogClient, we will now allow any daemon to
have any number of LogClients.  They may either all log to the default
facility and level, or they may see their facility and level specified
upon creation (via a new constructor).

This patch also changes 'handle_log_ack' in such a way that the LogClient
will handle all acks with bearing the LogClient's facility, or will
otherwise simply ignore them.  The function will return true whenever the
message has been handled or false if that was not the case.  It will fall
on the caller the responsibility of deciding whether the message will be
passed to other LogClients or not, and when it is to be release.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>