• Refactor listening socket list. All of the listening sockets are now maintained in a single linked-list in a transport-independent manner.
• Use queue.h for linked-list structure.
• Use linked-list for AllowedPeers.
• Use getaddrinfo(8) even for Unix Domain sockets.
• Use macros to type-casting from/to struct sockaddr{,_in,_in6}.
• Define fu_* macro for union f_un to shorten the member names.
• Remove an extra #include <sys/type.h>.
• Add "static" to non-exported symbols.
• !INET support is still incomplete but will be fixed later.

There is no functional change except for some minor debug messages.