- In _portable_ftell(), try fgetpos() before ftello() and ftell64().
  I ran into a situation on a 64-bit capable Linux where the C
  library's ftello() and ftell64() returned negative numbers despite
  fpos_t and off_t both being 64-bit types; fgetpos() did the right
  thing.

- Define a new typedef, Py_off_t, which is either fpos_t or off_t,
  depending on which one is 64 bits.  This removes the need for a lot
  of #ifdefs later on.  (XXX Should this be moved to pyport.h?  That
  file currently seems oblivious to large fille support, so for now
  I'll leave it here where it's needed.)