Reduce or remove dependency on profile.module

For sites that still use profile.module for building user profiles, the existing functionality is fine.

But some people either don't use it, or prefer to use nodeprofile to build more complex profiles.

If this module tracked user birthdate information separately from profile.module, that would also pave the way to have a setting that prevents users from just randomly changing their birthdate at will to lie about their age, or to make it possible to choose whether or not to make their birthdate publicly viewable, without having to install yet another module.

There are, of course, ways to get all this functionality without you having to change your module - it just requires several other modules to be installed (perhaps if you decide this feature request isn't something you want to pursue, the below will be helpful to other users).

Currently, I'm having to activate profile.module just so that I can have a birthdate field - I use nodeprofile for the actual user profiles on my site. Then I have to install "profile privacy" module so that users can decide whether or not to allow other users to see their birthday, and finally install "user Read-only" module so that I can keep users from simply altering their birthdate whenever they feel like it.