Add a hook for accessing profiles at IMCE load time

I am using IMCE (along with Plupload) as a bulk-file uploader for a custom entity type. The upload directory is dynamic - dependent upon the individual entity instance. In order to place the user in the proper directory when uploading, I found it necessary to modify the user's IMCE profile at runtime. I looked at several ways to do this, including stuffing a dynamic profile (and roles_profile) into the $conf array (or doing a variable_set() - but that would have beaucoup security issues). The best way I could find was to add an alter hook to allow the profiles and roles_profiles arrays to be modified just before use, in imce_user_profile(), optionally using on the active uri scheme as well.