Grabbed another chunk of code out of user_ops.c, purge_user()

Grabbed another chunk of code out of user_ops.c, purge_user()
This chunk checked if a user was logged in by testing the user numbers
It is now a function and exposed to the API as
int CtdlIsUserLoggedInByNum(int num)
Also marked the code with a FIXME since it is unsafe, a user could log in
after the test but before the delete and realy mess things up.