new unfollow command available for all players

This is the implementation of an "unfollow" command that will stop a player from following another PC or NPC.  It is a shortcut for longer commands such as 'follow self'.
This check for `ch->master` is redundant. We can simplify:

    if (ch->master) {
        if (AFF_FLAGGED(ch, AFF_CHARM)) {
            send_to_char(ch, "You feel compelled to follow %s\r\n", GET_NAME(ch->master));
        } else {
            stop_follower(ch);
        }
    } else {
        send_to_char(ch, "You are not following anyone.\r\n");
    }