Added checks to prevent PyUnicode_Count() from dumping core

Added checks to prevent PyUnicode_Count() from dumping core
in case the parameters are out of bounds and fixes error handling
for .count(), .startswith() and .endswith() for the case of
mixed string/Unicode objects.

This patch adds Python style index semantics to PyUnicode_Count()
indices (including the special handling of negative indices).

The patch is an extended version of patch #103249 submitted
by Michael Hudson (mwh) on SF. It also includes new test cases.