UNIX domain socket support (yhirose#1346)

* Add support UNIX domain socket

* `set_address_family(AF_UNIX)` is required

* add unittest for UNIX domain socket

* add support UNIX domain socket with abstract address

Abstract address of AF_UNIX begins with null(0x00) which can't be
delivered via .c_str() method.

* add unittest for UNIX domain socket with abstract address

Co-authored-by: Changbin Park <[email protected]>

Author: gh4ck3r