HeapTupleHeader accessor macros

This patch wraps all accesses to t_xmin, t_cmin, t_xmax, and t_cmax in
HeapTupleHeaderData in setter and getter macros called
HeapTupleHeaderGetXmin, HeapTupleHeaderSetXmin etc.

It also introduces a "virtual" field xvac by defining
HeapTupleHeaderGetXvac and HeapTupleHeaderSetXvac. Xvac is used by
VACUUM, in fact it is stored in t_cmin.

This patch doesn't change any behaviour, it just (hopefully) makes the
source code more readable. It is meant as a first step toward an
upcoming change in HeapTupleHeader, which will reduce on-disk tuple
size by four bytes.

Regression tests have been passed (except geometry, which always fails
on my machine). Please apply this patch (after my bitmaplen patch)
for 7.3.

It worked for 7.2 before I updated my cvs HEAD revision and recreated
the patch. If cosmetic changes are wanted for 7.2, I'll make a 7.2
version available.

Servus
Manfred