Fix new compiler warnings. Also boost "start" from (C) int to long and

Fix new compiler warnings.  Also boost "start" from (C) int to long and
return a (C) long:  PyArg_ParseTuple and Py_BuildValue may not let us get
at the size_t we really want, but C int is clearly too small for a 64-bit
box, and both the start parameter and the return value should work for
large mapped files even on 32-bit boxes.  The code really needs to be
rethought from scratch (not by me, though ...).