win32: (Temporarily) Add sockaddr_un definition from <afunix.h>

This is a temporary addition, until <afunix.h> ships with the
MinGW toolchain.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/360>

src/pulsecore/socket.h

@@ -11,6 +11,31 @@
 
 typedef long suseconds_t;
 
+/** Windows 10 supports AF_UNIX as of build 17603, with
+    support provided in the header file <afunix.h>.  However,
+    only the latest Windows SDK provides this file; older SDKs and
+    MinGW do not.
+
+    Hence we define SOCKADDR_UN here.  We do not expect this definition to change
+    as Windows has some pretty good binary backwards-compatibility guarantees.
+
+    This shouldn't pose a problem for older versions of Windows; we expect them to
+    fail with an error whenever we try to make a socket of type AF_UNIX. */
+#define UNIX_PATH_MAX 108
+
+typedef struct sockaddr_un
+{
+     ADDRESS_FAMILY sun_family;     /* AF_UNIX */
+     char sun_path[UNIX_PATH_MAX];  /* pathname */
+} SOCKADDR_UN, *PSOCKADDR_UN;
+
+#ifndef SUN_LEN
+#define SUN_LEN(ptr) \
+    ((size_t)(((struct sockaddr_un *) 0)->sun_path) + strlen((ptr)->sun_path))
+#endif
+
+#define HAVE_SYS_UN_H
+
 #endif
 
 #ifdef HAVE_WS2TCPIP_H