file system: Exception for non-permitted symlinks

Until now, there was not exception type for the condition where a
symlink was created on a file system w/o supporting symlinks, e.g., FAT.
The corresponding file-system server (ffat_fs) used to return a negative
handle as a work around. I added 'Permission_denied' to the list of
exceptions thrown by 'File_system::Session::symlink' to handle this case
in a clean way.

libports/src/server/ffat_fs/main.cc

@@ -290,7 +290,7 @@ namespace File_system {
 			Symlink_handle symlink(Dir_handle, Name const &name, bool create)
 			{
 				/* not supported */
-				return Symlink_handle(-1);
+				throw Permission_denied();
 			}
 
 			Dir_handle dir(Path const &path, bool create)

os/include/file_system_session/file_system_session.h

@@ -302,7 +302,7 @@ namespace File_system {
 		                 Dir_handle, Name const &, Mode, bool);
 		GENODE_RPC_THROW(Rpc_symlink, Symlink_handle, symlink,
 		                 GENODE_TYPE_LIST(Invalid_handle, Node_already_exists,
-		                                  Invalid_name, Lookup_failed),
+		                                  Invalid_name, Lookup_failed, Permission_denied),
 		                 Dir_handle, Name const &, bool);
 		GENODE_RPC_THROW(Rpc_dir, Dir_handle, dir,
 		                 GENODE_TYPE_LIST(Permission_denied, Node_already_exists,