Use PyLong_FromUnsignedLongLong() where needed.

There were a few places where PyLong_FromLong() was used for uint64_t
numbers. Properly use PyLong_FromUnsignedLongLong() there, and also
fix a few additional size/signedness issues while we're here.

Reported (and partial patch provided) by "The Count" on Bugzilla, thanks!

This fixes bug #1499.