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).

doc/fixes36.3

@@ -1,4 +1,4 @@
-$NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.94 $ $NHDT-Date: 1562838835 2019/07/11 09:53:55 $
+$NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.95 $ $NHDT-Date: 1564269131 2019/07/27 23:12:11 $
 
 This fixes36.3 file is here to capture information about updates in the 3.6.x
 lineage following the release of 3.6.2 in May 2019. Please note, however,
@@ -105,6 +105,8 @@ when farlook describes a monster at a visible spot as trapped, reveal the trap
 fix theft when poly'd into nymph form; 3.6.2 change made that anger the victim
 hero poly'd into nymph would steal gold along with other items
 change wizard mode #panic to require "yes" if 'paranoid_confirm:quit' is set
+add some additional validation checks when setting up a new timer (triggered
+	by an ancient source comment rather than by any observed problems)
 
 
 Fixes to Post-3.6.2 Problems that Were Exposed Via git Repository