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

Manfred Koizar