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.

Lib/test/string_tests.py

@@ -53,6 +53,10 @@ def run_method_tests(test):
 
     test('capitalize', ' hello ', ' hello ')
     test('capitalize', 'hello ', 'Hello ')
+
+    test('count', 'aaa', 3, 'a')
+    test('count', 'aaa', 0, 'b')
+    
     test('find', 'abcdefghiabc', 0, 'abc')
     test('find', 'abcdefghiabc', 9, 'abc', 1)
     test('find', 'abcdefghiabc', -1, 'def', 4)