Restore %self to refer to the fish pid

This brings back the %self argument. Like the original %self it only expands
if the argument is literally %self.

tests/expansion.err

@@ -8,6 +8,9 @@ fish: Invalid index value
 echo ()[d]
         ^
 
+####################
+# Percent self
+
 ####################
 # Catch your breath
 fish: $) is not a valid variable in fish.