htup and bufpage API clean up

Attached patch contains code cleanup in htup.h nad bufpage.[ch]. This patch is
part of pg upgrade project. Idea of this patch is to remove any direct access to
PageHeader and HeapTupleHeader. I already sent more complex patch including
mutli page version control last commitfest. This patch does not contain multi
page version control. I will send it separately.

Main changes in bufpage:
------------------------
- Remove PageGetItem/PageGetItemId. It has been replaced by PageGetHeapTuple and
PageGetIndexTuple
- Most macros are converted to functions. It will be necessary for following patch.


Main changes in htup:
---------------------
- htup.c is created
- Most macros HeapTupleHeader* are removed
- Add complete set of HeapTuple* functions
- Macros for "DatumTupleHeader" still remain, because they are use in PL

Other changes are related to the new API.

Several patches in a patch queue uses old API. It will require consolidation.

    Zdenek


--
Zdenek Kotala              Sun Microsystems
Prague, Czech Republic     http://sun.com/postgresql