Re-implement psql's input scanning to use a flex-generated lexer, as per

recent discussion.  The lexer is used for both SQL command text and
backslash commands.  The purpose of this change is to make it easier to
track the behavior of the backend's SQL lexer --- essentially identical
flex rules are now used by psql.  Also, this cleans up a lot of very
squirrelly code in mainloop.c and command.c.  The flex code is somewhat
bulkier than the removed code, but should be lots easier to maintain.