ecpg: add cross-checks to parse.pl for usage of internal tables.

ecpg: add cross-checks to parse.pl for usage of internal tables.

parse.pl contains several constant tables that describe tweaks
to be made to the backend grammar.  In the same spirit as
00b0e7204, add cross-checks that each table entry is used at
least once (or exactly once if that's appropriate).  This should
help catch cases where adjustments to the backend grammar cause
a table entry not to match as expected.

Per suggestion from Michael Paquier.

Discussion: https://postgr.es/m/ZsLVbjsc5x5Saesg@paquier.xyz