Implement getifaddrs.

This is a BSD extension also available in glibc, but is not POSIX.
Fixes #6279.

headers/compatibility/bsd/ifaddrs.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2015, Haiku, Inc. All Rights Reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _IFADDRS_H
+#define _IFADDRS_H
+
+
+struct ifaddrs {
+	struct ifaddrs  *ifa_next;    /* Next item in list */
+	const char      *ifa_name;    /* Name of interface */
+	unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
+	struct sockaddr *ifa_addr;    /* Address of interface */
+	struct sockaddr *ifa_netmask; /* Netmask of interface */
+	struct sockaddr *ifa_dstaddr;
+	#define         ifa_broadaddr ifa_dstaddr
+	void            *ifa_data;    /* Address-specific data */
+};
+
+
+int getifaddrs(struct ifaddrs **ifap);
+void freeifaddrs(struct ifaddrs *ifa);
+
+
+#endif