some grammar refactoring

Here is a series of patches to do some refactoring in the grammar around
the commands COMMENT, DROP, SECURITY LABEL, and ALTER EXTENSION ...
ADD/DROP. In the grammar, these commands (with some exceptions)
basically just take a reference to an object and later look it up in C
code. Some of that was already generalized individually for each
command (drop_type_any_name, drop_type_name, etc.). This patch combines
it into common lists for all these commands.

Advantages:

- Avoids having to list each object type at least four times.

- Object types not supported by security labels or extensions are now
explicitly listed and give a proper error message. Previously, this was
just encoded in the grammar itself and specifying a non-supported object
type would just give a parse error.

- Reduces lines of code in gram.y.

- Removes some old cruft.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services