Update mini_event.c

Update mini_event.c

When in heavy load, unbound opens many outside_network sockets for out going queries to delegation servers, which may result in a big fd(maxfd) value(for thread A 65500, for thread B 65501, for thread C ...).
There are situations when thread A has a max fd num 65500 where maxfd is of course 65500, thread B has max fd num 20 for now but maxfd is still 65501. Though linux kernel checks whether maxfd+1  passed by select syscall  is really the process' maxfd+1. Linux kernel can not tell maxfd+1 passed by thread B select syscall is much bigger(65501+1 or 65500+1  after trimed by kerne) than it should be (20+1).
In this situation, when kernel do_select() for thread B, much work is wasted.