User pointer validity checks.

Added routines that check whether a user pointer is accessible by the kernel,
and if not ask the pager to map-in those pages. I haven't implemented yet the
bit that asks the pager for paging-in.

include/l4/arch/arm/exception.h

@@ -85,4 +85,7 @@ static inline int in_user()
 	return !in_kernel();
 }
 
+int pager_pagein_request(unsigned long vaddr, unsigned long size,
+			 unsigned int flags);
+
 #endif