Fix a loop in C Mode caused by inadequate analysis of comments.

Fix a loop in C Mode caused by inadequate analysis of comments.

After M-;, and the insertion of the opening "/*", the CC Mode after-change
function got confused, since the new comment opener matched the end of a
subsequent comment, but moving back over that comment did not come back to the
starting point.  Fix this.

* lisp/progmodes/cc-engine.el (c-end-of-macro): Add a limit parameter, wherer
point is left if no end-of-macro is found before it.
(c-forward-sws): Change the `safe-start' mechanism.  Now `safe-start' is
non-nil except where we have an unclosed block comment at the end of a macro.
This enables us to populate the cache more fully, at the cost of some run
time.