user_access() needs a $reset parameter to support dynamic roles

I've contributed some code to the OG User Roles module. This module allows for roles to be assigned on a per-group basis. It works fairly simply: we pull in the roles that have been assigned to the current user in the current group in hook_init(). The problem is that the group context is available in hook_init(), so we have to try a load of heuristics to try to grab the gid from the URL. This is flaky and involves lots of hacks to support various modules. The reason we can't do this later is that user_access() statically caches the permissions a user has, and there's no $flush or $reset parameter to ask the function to get rid of its cache and recalculate permissions from $user->roles. Therefore as soon as user_access() is called adding roles to $user->roles becomes pointless.

I attach a patch against 6.0-beta which adds this parameter to user_access(), which would simplify and make more robust the OG User Roles plugin, as well as making user_access() more consistent with other caching functions like node_load().