More vacuum.c refactoring

Near the end of repair_frag() in vacuum.c -- under the comment /* clean
moved tuples from last page in Nvacpagelist list */ -- there is code
that marks itemids as unused. Itemids affected are those referring to
tuples that have been moved off the last page.

This code is very similar to vacuum_page(). The major difference is
that vacuum_page() uses vacpage->offsets while the code in repair_frag()
looks for MOVED_OFF bits in tuple headers. AFAICS the tuples with the
MOVED_OFF bit set are exactly those referenced by vacpage->offsets.

The attached patch passes make check and make installcheck. Please
apply unless I'm missing something.

Servus
Manfred