extensive header cleanup for standards conformance & correctness

thanks to Peter Mazinger (psm) for pointing many of these issues out
and submitting a patch on which this commit is loosely based

include/sys/socket.h

@@ -11,6 +11,7 @@ extern "C" {
 #define __NEED_uid_t
 #define __NEED_pid_t
 #define __NEED_gid_t
+#define __NEED_struct_iovec
 
 #include <bits/alltypes.h>