tevent: Fix maxfd calculation in tevent_select

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