include/inst: Implement instance status messages

include/inst: Implement instance status messages

In order to make the status of an instance visible to users and other
processes, functions are needed that can be used to set status messages
from within an instance.
This commit adds status messages for instances, which can be manipulated
through the following functions.

 - inst_set_status(): Set the status of the running instance
 - inst_get_status_message(): The the status message of an instance
 - inst_get_status_timestamp(): Get the timestamp of a status message
 - inst_get_status(): Get status message and timestamp of an instance