Resolve the ambiguity between user and role names as mentioned in MDEV-4397

MDEV-4397 (Roles) says that there is expected ambiguity between role names and user names, and also mentions a possible solution for that ("we can specify that a role name cannot match the first part of any user name."). It hasn't been done, please consider implementing it or something similar; the way it works now is indeed a bit confusing and somewhat disturbing, e.g.

create user manager;

grant all on db1.* to manager; # grant goes to the user manager@'%'

create role manager;

grant all on db2.* to manager; # grant goes to the role manager

etc.