Remove unnecessary zlib build feature dependency
BPositionIO: Add {Read,Write}AtExactly()

Analoguous to {Read,Write}Exactly(), just for the *At() versions.
Add private class BFdIO

Simple BPositionIO implementation using the POSIX API on a FD. In effect
similar to BFile, but more easily ported to kernel and boot loader (and
the FD is reusable).
boot loader: Add pwrite(), lseek(), ftruncate()

ftruncate() is just a stub (needed for BFdIO).
Switch package file accessor classes to use BPositionIO

* PackageFileHeap{Reader,Writer} as well as Package{Reader,Writer} and
  their implementation and super classes do now internally use a
  BPositionIO instead of a FD to access the package file. This provides
  more flexibility needed for features to come.
* BPackageReader has already grown a new Init() version with a
  BPositionIO* parameter.