Amend CC Mode to handle big C++ raw strings correctly.

Amend CC Mode to handle big C++ raw strings correctly.

Problems were caused by such a string spanning jit-lock chunks, and by a flaw
in the +-500 bytes boundaries imposed for macros.

* lisp/progmodes/cc-mode.el (c-extend-region-for-CPP): Check the +-500 byte
macro boundaries here.
(c-extend-font-lock-region-for-macros): Remove the check on the +-500 byte
lower boundary.  Fix the check on the upper boundary.

* lisp/progmodes/cc-fonts.el (c-font-lock-raw-strings): Handle the starting
point already being within a raw string.

* lisp/progmodes/cc-engine.el (c-raw-string-pos)
(c-depropertize-raw-strings-in-region, c-after-change-re-mark-raw-strings):
Modify regexp element "\\{,16\\}" to "\\{0,16\\}" for greater compatibility
with other Emacsen.