Patch by request at pgcon

A user at Pgcon made a pretty convincing case that it would sometimes
be handy to be able to see a list of columns in alphabetical order in
psql's \dt output. This is especially true when there are a very large
number of columns and you're looking for a specific column with name
matching some pattern in your head. Whie you could use your pager's
search feature, people are also quite accustomed to visually searching
for strings in sorted lists so that's a natural solution.

Adding such a feature would be pretty trivial, attached is a patch.

-- 
greg