Generalize handling of PD-session arguments

On Linux, we want to attach additional attributes to processes, i.e.,
the chroot location, the designated UID, and GID. Instead of polluting
the generic code with such Linux-specific platform details, I introduced
the new 'Native_pd_args' type, which can be customized for each
platform. The platform-dependent policy of init is factored out in the
new 'pd_args' library.

The new 'base-linux/run/lx_pd_args.run' script can be used to validate
the propagation of those attributes into core.

Note that this patch does not add the interpretation of the new UID and
PID attributes by core. This will be subject of a follow-up patch.

Related to #510.

base-codezero/include/base/native_types.h

@@ -130,6 +130,8 @@ namespace Genode {
 		 */
 		static addr_t context_virtual_size() { return 0x00100000UL; }
 	};
+
+	struct Native_pd_args { };
 }