Bring back former value of RPMSENSE_PREREQ

Bring back former value of RPMSENSE_PREREQ
- packages using the legacy PreReq still widely exist distros having
  patched PREREQ back in, we'll want to be able to deal with the legacy
  packages best we can, and to do that we need to be able to test for
  the prereq bit
- this also fixes ordering brokenness introduced by commit
  502f88e0e67bb1df91e09a32a370b04bda36259e all the way from 2004:
  with RPMSENSE_PREREQ equalling RPMSENSE_ANY, all co-requisites are seen
  as pre-requisites due to isLegacyPrefix() returning bogons (RhBug:502269)
- previously RPMSENSE_SCRIPT_PRE etc were also marked RPMSENSE_PREREQ to
  permit legacy rpm versions to deal the fine grained scriptlet dependencies
  somehow, this we dont want anymore