agent: enhance DeadEndState logic

[lucab]

Right now the agent keeps track at runtime of the deadend state (including the starting "unknown" case):

zincati/src/cincinnati/mod.rs

Lines 76 to 89 in 612955c

	impl DeadEndState {
	const FALSE: u8 = 0;
	const TRUE: u8 = 1;
	const UNKNOWN: u8 = 2;
	/// Return whether this is in a known dead-end state.
	pub fn is_deadend(&self) -> bool {
	self.0.load(Ordering::SeqCst) == Self::TRUE
	}
	/// Return whether this is in a known NOT dead-end state.
	pub fn is_no_deadend(&self) -> bool {
	self.0.load(Ordering::SeqCst) == Self::FALSE
	}

The no-deadend case (the most common one) can be further optimized by relying on the filesystem and avoid going through pkexec even once: if we determine that our fragment under /run/motd.d/ does not exist, there is no need to go through the "unset" logic.

On the other hand, the "is-a-deadend" case should be improved to account for the fact that the "reason" string could be updated between different graph scrapes. Thus the agent should internally cache the current deadend reason too, and refresh the MOTD fragment if the reason changed (even if the dead-end state stayed the same).