Check for overflow when converting from Python to unsigned integer types.

Python's PyArg_ParseTuple() unsigned integer type conversions don't
check for overflow, so stop using them.  Add bup_uint_from_py(),
bup_ulong_from_py(), and bup_ullong_from_py() functions that do check,
and use them instead.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>