Use macro to define the number of enum values

Use macro to define the number of enum values

Refactoring in the interest of code consistency, a follow-up to 2e068db56e31.

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