init: Fix init spinning once init.rc is complete

When the init process ran out of actions to execute, cur_action was
NULL and action_queue_empty() was true, but cur_command was still
set.  This patch clears cur_command when a new action is retreived,
and only depends on cur_action and action_queue_empty() to determine
the poll timeout.

Change-Id: Iaa95063c8c267a5b1ada9f20363b99c433e61ac4