Use correct POSIX offset-type for tell/seekdir

This fixes the FFI imports to use the proper `CLong` type over the
previous incorrect `COff` type, as using the wrong argument type can
cause problems when the `long` and `off_t` types have different size.

Historic note from the manual page:

  In glibc up to version 2.1.1, the return type of telldir() was off_t.
  POSIX.1-2001 specifies long, and this is the type used since glibc
  2.1.2 (released in 1999).