psql editor temp file extension

There was a question recently on a user list about how to configure an
editor to handle psql's editor temp files as SQL files.  While this is
doable with an advanced editor by reverse-engineering the file pattern
than psql uses, it would be easier if psql just added an extension
".sql".  Since both uses (\e and \ef) are actually SQL files, this
should not be a problem.  So I propose the attached patch.