Convert new_user_func_t to return void.

Convert new_user_func_t to return void.

This way, a function only has to worry about killing a user or not, rather
than doing that and then returning a corresponding value.

src/hash.h (new_user_func_t): Return void instead of int.
src/global.c (global_process_user): Likewise.
src/mod-blacklist.c (blacklist_check_user): Likewise.
src/mod-snoop.c (snoop_new_user): Likewise.
src/mod-sockcheck.c (sockcheck_new_user): Likewise.
src/nickserv.c (check_user_nick): Likewise.
  (handle_new_user): Delete.
  (init_nickserv): Register check_user_nick() directly.
src/opserv.c (opserv_new_user_check): Likewise.
src/proto-bahamut.c (AddUser): Do not check new_user_func return values.
src/proto-p10.c (AddUser): Likewise.