std::net: clamp linger timeout value to prevent silent truncation.

[devnexen]

Duration::as_secs() returns u64 but l_linger field type is narrower, c_int on most unix platforms, c_ushort on cygwin and windows. clamping before the cast, consistent with how set_timeout handles this.

[rustbot]

r? @joboet

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[joboet]

The implementation looks good to me, but I'll reassign this to libs-api to decide whether rounding down here is correct (as opposed to rounding the duration up and returning an error if the time is nor representable).

r? libs-api