fix all missing instances of __cplusplus checks/extern "C" in headers

patch by Arvid Picciani (aep)

include/arpa/inet.h

@@ -1,6 +1,10 @@
 #ifndef _ARPA_INET_H
 #define	_ARPA_INET_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <netinet/in.h>
 #include <inttypes.h>
 
@@ -30,4 +34,8 @@ int inet_aton (const char *, struct in_addr *); /* nonstandard but widely used *
 #define INET_ADDRSTRLEN  16
 #define INET6_ADDRSTRLEN 46
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif