Split elf dependency extraction out of librpmbuild to external helper

Split elf dependency extraction out of librpmbuild to external helper
- Elf dependency extraction code code lifted from rpmfcELF() and refactored
  to saner pieces. Having it in separate executable also frees librpmbuild
  of libelf dependency, clean up the unnecessary linkage etc from
  autofoo
- This lets internal dependency generator for elf files be
  overridden without losing file coloring (which is required for
  correct multilib handling). It also permits non-native elf files
  (eg when cross-building) to be handled by providing a custom
  elf dependency helper
- On the flip side, this inevitably slows down builds somewhat as
  two fork-exec's are needed for every elf file, but unlike invoking
  something like the python interpreter, this is a slim helper...
- All dependency extractors of the internal dependency generator are now
  external helpers (how twisted is that, huh? :) and thus can be customized
  and filtered through %__foo_provides|requires macros