Add an 'executable' flag to 'Rm_session::attach()'

With this patch clients of the RM service can state if they want a mapping
to be executable or not. This allows dataspaces to be mapped as
non-executable on Linux by default and as executable only if needed.

Partially fixes #176.

base-linux/include/base/platform_env.h

@@ -206,7 +206,8 @@ namespace Genode {
 					 **************************************/
 
 					Local_addr attach(Dataspace_capability ds, size_t size,
-					                  off_t, bool, Local_addr);
+					                  off_t, bool, Local_addr,
+					                  bool executable);
 
 					void detach(Local_addr local_addr);