Allow user to set and unset path property of variables

This adds flags --path and --unpath to builtin set, analogous to
--export and --unexport. These flags change whether a variable is
marked as a path variable.

Universal variables cannot yet be path variables.

tests/expansion.err

@@ -21,4 +21,4 @@ echo $$paren
 # Test tilde expansion
 
 ####################
-# Test delimiters
+# Test path variables