Get client process id over ip/port when server runs on UNIX socket. (yhirose#1418)

* handle socket options for UNIX socket same as others

 * set FD_CLOEXEC by default
 * invoke `socket_options` callback if set

* Offer Client info even on UNIX socket based Server

HTTP Request header "REMOTE_PORT" contains client process id if possible
when Server works on UNIX socket.

* retrigger checks

* retrigger checks

* add support macOS

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

Author: gh4ck3r