LZMA -> XZ renaming

LZMA -> XZ renaming
- what we support now in payloads is XZ, not the former LZMA format, rename
  user- and header-visible parts to match this
- rpmlib(PayloadIsXz) dependency versioned 5.2-1 to avoid unnecessary
  incompatibility with what rpm5 has and what Mandriva is already using
- only provide the rpmlib dependency if actually built with xz support
- rpmFileIsCompressed() attempts to guess the magicless old lzma format
  by file suffix, so it can handle both right now