timer validations

timer validations

Add the contributed code that checks for attempting to start a
duplicate timer.  It's based on a comment which must have been there
at least 25 years and doesn't solve any known problems, but it is
conceptually similar to the large amount of sanity checking which has
gone into 3.6.x.

It didn't work as is because it was comparing two unions with '=='.
I don't know offhand whether C++ supports that but C doesn't (through
C11 at least; don't know about C17).  The union ('anything') is simple
enough that two instances can be compared without jumping through hoops.

I've also added another check for timer 'kind' (level, object, monster,
or global).