Enable custom socket end points and allow Unix Domain Sockets

[IlyaBiryukov]

With code separation into System.Net.Primitives and System.Net.Sockets,
EndPoint extensibility was broken because System.Net.Sockets started to
use its own copy of SocketAddress and didn't respect SocketAddress that
a custom EndPoint may provide.

The fix is to allow conversion between SocketAddress from System.Net.Primitives
and System.Net.Sockets. This way custom implementations of EndPoint will
be able to provide their own SocketAddress and it'll be honored by the
Socket APIs.

The fix also allows sockets to use 'Unspecified' protocol type which is
needed for Unix Domain Sockets. There are several changes in socket test
server to allow tests pass protocol type.

Add new unit tests that use end point extensibility to implement Unix
Domain Sockets.

Fix #4777

[dnfclas]

Hi @IlyaBiryukov, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

In order for us to evaluate and accept your PR, we ask that you sign a contribution license agreement. It's all electronic and will take just minutes. I promise there's no faxing. https://cla2.dotnetfoundation.org.

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[IlyaBiryukov]

I've just associated my GitHub account with Microsoft before creating this pull request, I guess the DNFBOT doesn't know that yet and added cla-required tag.

[davidsh]

Even if you associate your GitHub account with your Microsoft account, you still need to sign the CLA.

[davidsh]

cc: @CIPop @pgavlin

[IlyaBiryukov]

cc: @AArnott

[Maxwe11]

I've just associated my GitHub account with Microsoft before creating this pull request, I guess the DNFBOT doesn't know that yet and added cla-required tag.

Commit signed off by @ilyab which is not associated with Microsoft 😄

[IlyaBiryukov]

Fixed the commit author.

[dnfclas]

@IlyaBiryukov, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[pgavlin]

What version of OS X are you running? On 10.9, the max path length is 104: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man4/unix.4.html

[IlyaBiryukov]

108 is on Linux. OSX has 104. So we should make it 104 I guess..

[IlyaBiryukov]

Docs say that it may even be as less as 92, for example here, http://man7.org/linux/man-pages/man7/unix.7.html. I'll change it to 92.

[pgavlin]

Sounds good. FWIW, the Open Group Base Specification is generally a good source of truth for these things, and it agrees with 92 as a lower bound.

[AArnott]

LGTM

[IlyaBiryukov]

If there are no more comments, how can this pull request be committed?

[CIPop]

How would this work for a DnsEndPoint? Please add a test.

[IlyaBiryukov]

There are already existing tests for DnsEndpoint. The code checks if it's DNS end point before it calls IPEndPointExtensions.

[IlyaBiryukov]

I addressed all the comments. Is there anything else?

[pgavlin]

Nit: we might want this to be in a finally block to ensure that we run cleanup even if the test fails.

[IlyaBiryukov]

Sounds good.

[pgavlin]

LGTM aside from one nit. Thanks for doing this!