Declare variable-length catalog columns as [] rather than [1]

Variable-length catalog columns have been declared like

text attoptions[1];

but that "1" has always been a fiction. Before the use of #ifdef
CATALOG_VARLEN, these declarations were visible to the C compiler, and
this was also before flexible array members were universally available,
so this was just a convenient workaround to make this compile. But
these reasons are long gone, and the "1" is now just a confusing relic.
Change this to

text attoptions[];

which more intuitively reflects the actual nature of these fields (while
still being syntactically valid but semantically invalid C code).

Catalog.pm could already parse both spellings, but no existing code used
bare []. To enforce future consistency, it is changed to no longer
permit digits between the brackets.

(Obviously, catalog definitions are not backpatched, so this shouldn't
create any new maintenance burden.)