Handle partial reads and errors in package IO (RhBug:802839, RhBug:580974)

Handle partial reads and errors in package IO (RhBug:802839, RhBug:580974)

- There are no guarantees Fread() will return all of the requested size:
  it can return partial data eg on signals and pipe descriptors. Introduce
  a helper function to handle this centrally for all package read IO,
  effectively reintroducing timedRead() but without the caveats:
  timedRead() did not work on compressed streams, did not handle
  eg EINTR correctly and while really being an internal helper,
  was exported in the API.
(cherry picked from commit cbd6ef58bbc122e6adf2138679915bd3845d6756)