pyexpat code cleanup and minor refactorings:

The handlers array on each parser now has the invariant that None will
never be set as a handler; it will always be NULL or a Python-level
value passed in for the specific handler.

have_handler():  Return true if there is a Python handler for a
    particular event.

get_handler_name():  Return a string object giving the name of a
    particular handler.  This caches the string object so it doesn't
    need to be created more than once.

get_parse_result():  Helper to allow the Parse() and ParseFile()
    methods to share the same logic for determining the return value
    or exception state.

PyUnknownEncodingHandler(), PyModule_AddIntConstant():
    Made these helpers static.  (The later is only defined for older
    versions of Python.)

pyxml_UpdatePairedHandlers(), pyxml_SetStartElementHandler(),
pyxml_SetEndElementHandler(), pyxml_SetStartNamespaceDeclHandler(),
pyxml_SetEndNamespaceDeclHandler(), pyxml_SetStartCdataSection(),
pyxml_SetEndCdataSection(), pyxml_SetStartDoctypeDeclHandler(),
pyxml_SetEndDoctypeDeclHandler():
    Removed.  These are no longer needed with Expat 1.95.x.

handler_info:
    Use the setter functions provided by Expat 1.95.x instead of the
    pyxml_Set*Handler() functions which have been removed.

Minor code formatting changes for consistency.
Trailing whitespace removed.