libc/exit: Purge calls to userspace API exit() from kernel

Remove calls to the userspace API exit() from the kernel. The problem
with doing such calls is that the exit functions are called with kernel
mode privileges which is a big security no-no.

libs/libc/stdlib/lib_exit.c

@@ -31,6 +31,8 @@
 #include <stdlib.h>
 #include <unistd.h>
 
+#ifndef __KERNEL__
+
 /****************************************************************************
  * Private Data
  ****************************************************************************/
@@ -148,3 +150,5 @@ void _Exit(int status)
 {
   _exit(status);
 }
+
+#endif /* __KERNEL__ */