Fix PyTokenizer_FindEncoding() for OS X 10.4. Turns out that seeking to the

beginning of a file through a file pointer is not reflected when reading from a
file descriptor.  Using both fflush() and fpurge() does not solve it.  One must
use lseek() directly on the file descriptor to get the desired effect.

This might suggest that we standardize on either file pointers (FILE) or file
descriptors (int) for all C code used.