CC Mode: Fontify unbalanced quotes in unconstrained multiline strings, etc.

CC Mode: Fontify unbalanced quotes in unconstrained multiline strings, etc.

("Unconstrained" meaning that every string is multiline, without needing such
special marking as used by Pike Mode.)

* lisp/progmodes/cc-mode.el (c-pps-to-string-delim): Don't process the char
before BOB.
(c-multiline-string-check-final-quote): New function.
(c-bc-changed-stringiness): New variable.
(c-before-change-check-unbalanced-strings): Add handling for unconstrained
multiline strings.
(c-after-change-re-mark-unbalanced-strings): Add handling for unconstrained
multiline strings.  Handle escaped double quotes more accurately.