Start adding POSIX 1.e draft file capability support for real

Start adding POSIX 1.e draft file capability support for real
- Parse %caps() from spec filelists, making best-guess verification of
  capability string sanity by passing to cap_from_text()
- The posix draft specifies capability export presentation through
  cap_copy_ext() which would be fine, except that we don't have support for
  arrays of binary data. So we simply store the textual representation
  of the capabilities in a string array which we do have.
- Only add capability tag on packages which actually have capabilities to
  avoid unnecessary header bloat.
- Add a new rpmlib() dependency for file capabilities, packages relying
  on file capabilities wont work correctly unless the capabilities are set.
  To be fully correct, support for on-filesystem file capabilities should
  be checked at runtime, as this depends on kernel versions and such...