_MINIX_SYSTEM - fixes for in-minix crossbuild

. define _MINIX_SYSTEM for all system code from minix.service.mk
	. hide some system-level declarations and definitions
	  behind _MINIX_SYSTEM to cleanly fix host tool build problems on
	  Minix (such as: NONE being defined and paddr_t being used but not
	  declared)
	. the similar definition _SYSTEM is unsuitable as it changes the
	  values of errno definitions

Change-Id: I407de79e2575115243a074b16e79546a279cfa3e

commands/ps/ps.c

@@ -39,6 +39,8 @@
  *	  user space and converted into a concatenated argument list).
  */
 
+#define _MINIX_SYSTEM 1
+
 #include <minix/config.h>
 #include <minix/endpoint.h>
 #include <minix/paths.h>

commands/service/parse.c

@@ -1,4 +1,6 @@
 
+#define _MINIX_SYSTEM 1
+
 #include <stdarg.h>
 #include <assert.h>
 #include <stdlib.h>

commands/svrctl/svrctl.c

@@ -1,3 +1,5 @@
+#define _MINIX_SYSTEM 1
+
 #include <sys/svrctl.h>
 #include <sys/types.h>
 #include <ctype.h>