Implemented the missing part in preserve-libs for Mach-O systems.

Implemented the missing part in preserve-libs for Mach-O systems.
Unfortunately we have to call otool twice for each lib we have
preserved.  Fortunately, it is to be expected that the number of
preserved libs remains low.  Still avoiding the calls to otool and
scanelf at this stage would be really preferable, especially since bash
can do the magic we need to do here a lot easier than python can.

svn path=/main/branches/prefix/; revision=10737