Hide 'struct ucred' behind '#ifdef _KERNEL', this should stop userland

from attempting to use it for good.  There is a catch, kvm_proc.c needs
to '#define _KERNEL' to get at the ucred.

Requested by: rwatson

lib/libkvm/kvm_proc.c

@@ -52,6 +52,9 @@ static char sccsid[] = "@(#)kvm_proc.c	8.3 (Berkeley) 9/23/93";
  */
 
 #include <sys/param.h>
+#define _KERNEL
+#include <sys/ucred.h>
+#undef _KERNEL
 #include <sys/user.h>
 #include <sys/proc.h>
 #include <sys/exec.h>

sys/sys/ucred.h

@@ -49,6 +49,7 @@
  * Please do not inspect cr_uid directly to determine superuserness.
  * Only the suser() or suser_cred() function should be used for this.
  */
+#ifdef _KERNEL
 struct ucred {
 	u_int	cr_ref;			/* reference count */
 #define	cr_startcopy cr_uid
@@ -68,6 +69,7 @@ struct ucred {
 };
 #define	NOCRED	((struct ucred *)0)	/* no credential available */
 #define	FSCRED	((struct ucred *)-1)	/* filesystem credential */
+#endif /* _KERNEL */
 
 /*
  * This is the external representation of struct ucred.