Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands:\quit_if, \quit_unless)

The check I was suggesting on whether Ctrl+C has been pressed
on an empty line seems harder to implement, because get_interactive()
just calls readline() or fgets(), which block to return when a whole
line is ready. AFAICS psql can't know what was the edit-in-progress
when these functions are interrupted by a signal instead of
returning normally.
But I don't think this check is essential, it could be left to another patch.

Glad I wasn't missing something obvious.
I suppose we could base the behavior on whether there's at least one full line already buffered.
However, I agree that it can be left to another patch.