network: Use kernel/user space header files

Add and use <machine/rtems-bsd-kernel-space.h> and
<machine/rtems-bsd-user-space.h> similar to the libbsd to avoid command
line defines and defines scattered throught the code base.

Simplify cpukit/libnetworking/Makefile.am.

Update #3375.

bsps/shared/net/dec21140.c

@@ -35,7 +35,7 @@
  *
  */
 
-#define __INSIDE_RTEMS_BSD_TCPIP_STACK__
+#include <machine/rtems-bsd-kernel-space.h>
 
 #include <rtems.h>
 #include <inttypes.h>