flagutil: move REVIDX_KNOWN_FLAGS source of truth in flagutil (API)...
flagutil: move REVIDX_KNOWN_FLAGS source of truth in flagutil (API)
Since REVIDX_KNOWN_FLAGS is "not really a constant" (extension can update it)
and python integer,... it needs to be the responsability of a single module and
always accessed through the module. We update all the user and move the source
of truth in flagutil.
