ctdb-logging: Rework debug level parsing

ctdb-logging: Rework debug level parsing

Put declarations into ctdb_logging.h, factor out some common code,
clean up #includes.

Remove the check so see if the 1st character of the debug level is
'-'.  This is wrong, since it is trying to check for a negative
numeric debug level (which is no longer supported) and would need to
be handled in the else anyway.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>