procd: instance: Support deleting stopped instances

procd: instance: Support deleting stopped instances

procd currently does not support deleting a stopped instance. The reason
is that we return in instance_stop(), if pending is set to false. This
patch adds a new function, instance_delete(), which does the necessary
clean-up of an instance. instance_delete() is called before we return in
instance_stop(), as well as when a process that should not be restarted
has exited in instance_exit().

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>