Make sure unlock is called when 'global_mutex' reaches zero count. Add verbose
variable in order to disable some output. Disable irritating 'Overflow' messages
in 'sys_mbox_post' and 'sys_mbox_try_post' per default. This may happen and is
not an error, since the ring buffer is full and will be emptied eventually.
Remove priority from genode_org run script.
Should fix#347