Avoid appending a user to curr_opers or curr_helpers more than once.

Avoid appending a user to curr_opers or curr_helpers more than once.

hash.h (userList_contains): Declare new function.
hash.c (userList_contains): Implement it.
nickserv.c (set_user_handle_info): Use it to check against curr_helpers.
proto-p10.c (mod_usermode): Use it for curr_opers.