Add option to call a scenario special node to apply the effect of a trap

Also:
- The possible traps resulting from "random trap" is expanded to include the knife trap
- A custom picture can be specified for the trap dialog
- A given custom trap special node can be reused for the same trap with different levels - the pointer -5 holds the trap level passed to the ONCE_TRAP node

rsrc/strings/specials-text-once.txt

@@ -212,13 +212,13 @@ Stuff Done Flag Part B
 First part of message
 Second part of message
 Unused
-Unused
-Unused
+Picture number
+Picture type
 Type of trap
 Trap severity (0 .. 3)
 Unused
 Penalty (0 .. 100, higher is harder)
-Unused
+(Custom only) Special node for effect
 Unused
 Special after trap finished
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