Prevent memory leaks from accumulating across printtup() calls.

Historically, printtup() has assumed that it could prevent memory leakage
by pfree'ing the string result of each output function and manually
managing detoasting of toasted values.  This amounts to assuming that
datatype output functions never leak any memory internally; an assumption
we've already decided to be bogus elsewhere, for example in COPY OUT.
range_out in particular is known to leak multiple kilobytes per call, as
noted in bug #8573 from Godfried Vanluffelen.  While we could go in and fix
that leak, it wouldn't be very notationally convenient, and in any case
there have been and undoubtedly will again be other leaks in other output
functions.  So what seems like the best solution is to run the output
functions in a temporary memory context that can be reset after each row,
as we're doing in COPY OUT.  Some quick experimentation suggests this is
actually a tad faster than the retail pfree's anyway.

This patch fixes all the variants of printtup, except for debugtup()
which is used in standalone mode.  It doesn't seem worth worrying
about query-lifespan leaks in standalone mode, and fixing that case
would be a bit tedious since debugtup() doesn't currently have any
startup or shutdown functions.

While at it, remove manual detoast management from several other
output-function call sites that had copied it from printtup().  This
doesn't make a lot of difference right now, but in view of recent
discussions about supporting "non-flattened" Datums, we're going to
want that code gone eventually anyway.

Back-patch to 9.2 where range_out was introduced.  We might eventually
decide to back-patch this further, but in the absence of known major
leaks in older output functions, I'll refrain for now.