Net result of Tim's checkins to stropmodule.c (2.78, 2.79, 2.80, 2.81),

Net result of Tim's checkins to stropmodule.c (2.78, 2.79, 2.80, 2.81),
stringobject.c (2.114, 2.115) and test_strop.py (1.11, 1.12). Fixes
'replace' behaviour on systems on which 'malloc(0)' returns NULL (together
with previous checkins) and re-synchs the string-operation code in
stringobject.c and stropmodule.c, with the exception of 'replace', which has
the old semantics in stropmodule but the new semantics in stringobjects.