Include arpa/inet.h, since that's where htons() and friends are

supposed to be defined according to XPG4.2.
Found by Evan <n2xjk@ulster.net> for the MVS platform.

e_os.h

@@ -355,12 +355,14 @@ extern HINSTANCE _hInstance;
 #    if defined(VMS) && !defined(__DECC)
 #      include <socket.h>
 #      include <in.h>
+#      include <inet.h>
 #    else
 #      include <sys/socket.h>
 #      ifdef FILIO_H
 #        include <sys/filio.h> /* Added for FIONBIO under unixware */
 #      endif
 #      include <netinet/in.h>
+#      include <arpa/inet.h>
 #    endif
 
 #    if defined(NeXT) || defined(_NEXT_SOURCE)