priv: use unistd.h to get `close()`
event: don't call `netsnmp_check_outstanding_agent_requests()`

We don't use delegated requests and therefore, this call is not
necessary. It was put here out of safety but it seems that it can
trigger spurious alarms and microseconds timeout. This makes lldpd
consume a lot of CPU. We just remove this call from the event loop.