Add socket extensions for cygwin

[Berrysoft]

r? @joboet

• Abstract name uds addr
• quickack
• passcred

[Berrysoft]

Some limitations:

• Cannot connect the addr from recvfrom of a unix datagram socket.
  • It seems like a bug of cygwin.
  • A patch has been proposed.
  • UPDATE: patch merged. Now the test passes.
• Cannot bind to abstract addr - not supported by cygwin.
• Unbounded unix socket has an abstract local addr - a feature (autobind?) of cygwin.
• Unix socket doesn't support recvmsg with control data. It fallbacks to recvmsg without control data.
• Unix socket connect blocks until the server accepts it. See mailing list. I have to ignore some tests because of that.

[bors]

☔ The latest upstream changes (presumably #143157) made this pull request unmergeable. Please resolve the merge conflicts.