pkgbinarymangler: Disable running pkgstripfiles by default

Running pkgstripfiles is only useful when pulling packages from non-Ubuntu and non-PPA repos. Otherwise, it must have already been run when importing the package into Ubuntu. In addition pkgstripfiles slows down compilation of packages, specially when multiple packages are built from the same source or there are PNG optimizations to be run.

This should probably speed up the compilation of multiple packages and might help with !386 (merged)