Bug 975996: Add _PyTime_DoubleToTimet to C API

New include file timefuncs.h exports private API function
_PyTime_DoubleToTimet() from timemodule.c.  timemodule should export
some other functions too (look for painful bits in datetimemodule.c).

Added insane-argument checking to datetime's assorted fromtimestamp()
and utcfromtimestamp() methods.  Added insane-argument tests of these
to test_datetime, and insane-argument tests for ctime(), localtime()
and gmtime() to test_time.

Include/timefuncs.h

@@ -0,0 +1,23 @@
+/*  timefuncs.h
+ */
+
+/* Utility function related to timemodule.c. */
+
+#ifndef TIMEFUNCS_H
+#define TIMEFUNCS_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Cast double x to time_t, but raise ValueError if x is too large
+ * to fit in a time_t.  ValueError is set on return iff the return
+ * value is (time_t)-1 and PyErr_Occurred().
+ */
+PyAPI_FUNC(time_t) _PyTime_DoubleToTimet(double x);
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif  /* TIMEFUNCS_H */