vacuum.c refactoring

. rename variables
. cur_buffer -> dst_buffer
. ToPage -> dst_page
. cur_page -> dst_vacpage
. move variable declarations into block where variable is used
. various Asserts instead of elog(ERROR, ...)
. extract functionality from repair_frag() into new routines
. move_chain_tuple()
. move_plain_tuple()
. update_hint_bits()
. create type ExecContext
. add comments

This patch does not intend to change any behaviour. It passes make
check, make installcheck and some manual tests. It might be hard to
review, because some lines are affected by more than one change. If
it's too much to swallow at once, I can provide it in smaller chunks ...

Servus
Manfred