Optimise c-parse-state for large buffers with few (if any) braces.

Optimise c-parse-state for large buffers with few (if any) braces.

* lisp/progmodes/cc-engine (c-get-fallback-scan-pos): Search a maximum of
50,000 characters back for the two BODs.  Return nil if we dont' find them.
(c-parse-state-get-strategy): For strategy `forward', always use the position
`good-pos' for `start-point', even when there's a change of current macro.
Deal with a possible return value of nil from c-get-fallback-scan-pos (as
above).
(c-invalidate-state-cache-1): For `c-state-cache-good-pos', instead of
sometimes using the minimum scan pos (leading to extensive scanning of the
entire buffer) use a point close to `here'.