tevent: Fix maxfd calculation in tevent_select
When doing
fd1 = tevent_add_fd(ev, ev, 2, 0, NULL, NULL);
fd2 = tevent_add_fd(ev, ev, 3, 0, NULL, NULL);
TALLOC_FREE(fd2);
fd2 = tevent_add_fd(ev, ev, 1, 0, NULL, NULL);
we end up with select_ev->maxfd==1. This is wrong.
An alternative fix might be to make select_ev->maxfd an unsigned int and make
EVENT_INVALID_MAXFD==UINT_MAX. But in theory we might end up with an fd of
UINT_MAX.
std_event_add_fd() contains exactly the same piece of code, so I'm directly
pushing it.
Volker