Use macro to define the number of enum values

Refactoring in the interest of code consistency, a follow-up to 2e068db56e.

The argument against inserting a special enum value at the end of the enum
definition is that a switch statement might generate a compiler warning unless
it has a default clause.

Aleksander Alekseev, reviewed by Michael Paquier, Dean Rasheed, Peter Eisentraut

Discussion: https://postgr.es/m/CAJ7c6TMsiaV5urU_Pq6zJ2tXPDwk69-NKVh4AMN5XrRiM7N%2BGA%40mail.gmail.com

src/backend/postmaster/autovacuum.c

@@ -247,9 +247,10 @@ typedef enum
 {
 	AutoVacForkFailed,			/* failed trying to start a worker */
 	AutoVacRebalance,			/* rebalance the cost limits */
-	AutoVacNumSignals,			/* must be last */
 }			AutoVacuumSignal;
 
+#define AutoVacNumSignals (AutoVacRebalance + 1)
+
 /*
  * Autovacuum workitem array, stored in AutoVacuumShmem->av_workItems.  This
  * list is mostly protected by AutovacuumLock, except that if an item is