Refactor ecpg grammar so that it uses the core grammar's unreserved_keyword

Refactor ecpg grammar so that it uses the core grammar's unreserved_keyword
list, minus a few specific words that have to be treated specially.  This
replaces a hard-wired list of keywords that would have needed manual
maintenance, and was not getting it.  The 8.4 coding was already missing
these words, causing ecpg to incorrectly treat them as reserved words:
CALLED, CATALOG, DEFINER, ENUM, FOLLOWING, INVOKER, OPTIONS, PARTITION,
PRECEDING, RANGE, SECURITY, SERVER, UNBOUNDED, WRAPPER.  In HEAD we were
additionally missing COMMENTS, FUNCTIONS, SEQUENCES, TABLES.
Per gripe from Bosco Rama.