Fix infinite trigger count not interacting correctly with "+Interval:"

[MageKing17]

There's an undocumented feature whereby negative trigger counts are treated as infinite. However, since it doesn't use the bump_timestamp system of positive trigger counts, it'll behave more like an infinite repeat count (e.g. with "+Interval: 5" it will only check every 5 seconds instead of becoming active again after 5 seconds have passed). This just makes negative values set bump_timestamp and excludes them from resetting the timestamp if the event didn't trigger that frame (checking MEF_USING_TRIGGER_COUNT so that the other behavior is still possible if you set a negative repeat_count and don't specify a trigger_count).

(As an addendum, FRED makes it somewhat more difficult to use negative repeat and trigger counts because it doesn't allow typing non-digit characters; you can still paste in a "-", but FRED will display e.g. 4294967295 instead of -1, even though it will save -1 in the mission file. This PR just fixes FSO's interaction with infinite trigger counts, not FRED's interface issues with them.)

[MageKing17]

For those who are curious, this is the stray commit that accidentally wound up in PR #41; removing it from there reminded me that I should probably make a PR for it.

[Karajorma]

I'm not entirely sure this is an undocumented feature so much as a bug in FRED actually. Repeat count is frequently set to -1 and that is not an indication that it is meant to loop infinitely. I don't have time to look at how this is meant to work but there is a good chance that FRED occasionally also does this with trigger count, and if so, this change has just broken it. I'll look into it later today.

[MageKing17]

There are comments that explicitly state that a repeat count of -1 means "infinite". While this is not inherently the case with trigger count (if memory serves, it defaults to -1), that's what the MEF_USING_TRIGGER_COUNT flag check is for.

As I'm unaware of any missions that actually specify a negative trigger count (everyone seems to just use 9999 or a billion or some other large positive number), I'm unaware of how this would break anything.

[Karajorma]

Yeah, ran some checks and it does appear that it was repeat count that I've seen set to -1 multiple times and never trigger count. So this change should be safe.