[PATCH] Add SIGCHLD catch to psql

Greetings,

Toying around with FETCH_COUNT today, I discovered that it didn't do
the #1 thing I really wanted to use it for- query large tables without
having to worry about LIMIT to see the first couple hundred records.
The reason is simple- psql ignores $PAGER exiting, which means that it
will happily continue pulling down the entire large table long after
you've stopped caring, which means you still have to wait forever.

The attached, admittedly quick hack, fixes this by having psql catch
SIGCHLD's using handle_sigint. I've tested this and it doesn't
appear to obviously break other cases where we have children (\!, for
example), since we're not going to be running a database query when
we're doing those, and if we are, and the child dies, we probably want
to *stop* anyway, similar to the $PAGER issue.

Another approach that I considered was fixing various things to deal
cleanly with write's failing to $PAGER (I presume the writes *were*
failing, since less was in a defunct state, but I didn't actually
test). This solution was simpler, faster to code and check, and alot
less invasive (or so it seemed to me at the time).

Anyway, this makes FETCH_COUNT alot more useful, and, in my view, the
current behaviour of completely ignoring $PAGER exiting is a bug.

Thanks,

Stephen