Import additional networking headers from NetBSD

While still a small subset of the NetBSD headers, this set should
allow various additional utilities to be compiled without too many
MINIX3-specific

Import additional networking headers from NetBSD

While still a small subset of the NetBSD headers, this set should
allow various additional utilities to be compiled without too many
MINIX3-specific changes (even if those utilities will not yet work).

Change-Id: Idc70e9901d584e960cd406f75f561dcc9a4ddb7d

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