MINOR: proxy: bit field for proxy_find_best_match diff status

MINOR: proxy: bit field for proxy_find_best_match diff status

function proxy_find_best_match can update the caller by updating an int
provided in argument.
For now, proxy_find_best_match hardcode bit values 0x01, 0x02 and 0x04,
which is not understandable when reading a code exploiting them.

This patch defines 3 macros with a more explicit wording, so further
reading of a code exploiting the magic bit values will be understandable
more easily.