Just use C89 size_t instead of rolling our own Size_t & Off_t

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Use SEEK_* names instead of raw numbers for fseek whence argument

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>