rely on standard POSIX headers to fix globs of warnings

fixes that overlap with the ipv6 patch

--- a/error.h
+++ b/error.h
@@ -1,7 +1,7 @@
 #ifndef ERROR_H
 #define ERROR_H
 
-extern int errno;
+#include <errno.h>
 
 extern int error_intr;
 extern int error_nomem;
--- a/dns_random.c
+++ b/dns_random.c
@@ -1,3 +1,4 @@
+#include <unistd.h>
 #include "dns.h"
 #include "taia.h"
 #include "uint32.h"
--- a/dns_rcrw.c
+++ b/dns_rcrw.c
@@ -1,3 +1,4 @@
+#include <unistd.h>
 #include "taia.h"
 #include "env.h"
 #include "byte.h"
--- a/dns_transmit.c
+++ b/dns_transmit.c
@@ -1,3 +1,4 @@
+#include <sys/socket.h>
 #include "socket.h"
 #include "alloc.h"
 #include "error.h"