Add Unix domain socket support.

[ghost]

Updates:

• Add support for Unix domain sockets in hsocket.
  • Rearranged some code so that many of them can be reused, reducing boilerplates.
• Add Unix stream/datagram socket server/client API in hloop.
• Add Unix domain socket support in server/client examples.
  • tcp.c and udp.c examples renamed to stream_server.c and dgram_server.c, and by specifying --unix argument, the server will bind to Unix domain socket instead of IP ports.
  • Add Unix domain socket client support in nc.c (-U and -D arguments).
• Update README documentation for the new features.

Notes:

• Although Windows began supporting Unix domain sockets as of late 2018, it is not supported yet in this pull request.
• This feature is not enabled by default, change ENABLE_UDS=yes in config.mk to enable it.

[ithewei]

Thank you for contribution, I've added support for Unix Domain Socket based on your PR.
But I only add it to hsocket and will not add to hloop ( haccept、hconnect、hread、hwrite、hclose can pass fd directly )