move and improve SA_LEN emulation

If sa_family is not set, sa_len should be zero.

include/Makefile.am

@@ -1,8 +1,8 @@
 noinst_HEADERS =
 noinst_HEADERS += signal.h
 noinst_HEADERS += unistd.h
+noinst_HEADERS += netinet/in.h
 noinst_HEADERS += sys/types.h
-noinst_HEADERS += sys/socket.h
 noinst_HEADERS += sys/time.h
 noinst_HEADERS += imsg.h
 noinst_HEADERS += string.h

include/netinet/in.h

@@ -0,0 +1,24 @@
+/*
+ * Public domain
+ * netinet/in.h compatibility shim
+ */
+
+#include_next <netinet/in.h>
+
+#ifndef SA_LEN
+static inline socklen_t sockaddr_len(void *a)
+{
+	struct sockaddr *sa = a;
+	switch (sa->sa_family) {
+	case AF_INET:
+		return sizeof(struct sockaddr_in);
+	case AF_INET6:
+		return sizeof(struct sockaddr_in6);
+	case AF_UNSPEC:
+		return sizeof(struct sockaddr);
+	default:
+		return 0;
+	}
+}
+#define SA_LEN(X) sockaddr_len(X)
+#endif

include/sys/socket.h

@@ -1,12 +0,0 @@
-/*
- * Public domain
- * sys/socket.h compatibility shim
- */
-
-#include_next <sys/socket.h>
-
-#ifndef SA_LEN
-#define SA_LEN(X) \
-	(((struct sockaddr*)(X))->sa_family == AF_INET ? sizeof(struct sockaddr_in) : \
-	 ((struct sockaddr*)(X))->sa_family == AF_INET6 ? sizeof(struct sockaddr_in6) : sizeof(struct sockaddr))
-#endif