Attempt to align this more with trunk

Attempt to align this more with trunk
- reshuffle qa_check code using functions such that the order is the
  same as in trunk, but with some extra offsets
  (at the same time avoid running QA checks for platforms other than the
  current one)
- split out elf, macho, pecoff, xcoff, misc and prefix qa code blocks,
  keep a bit of general (misc) checks in the original qa_check func for
  the point mentioned above
- only chdir to ${D}, not ${ED}, this has the advantage that if that
  fails we're on the same panic level as normal Portage, and that we no
  longer need default src_install implementations that just create
  ${ED} to avoid a failing QA check, like bug #290245

svn path=/main/branches/prefix/; revision=14746