Argument Parsing

- Eliminated stored arg_overrides and file_override in Context.
- Removed Filter, Subst, Task, Sequence from Context.
- Remove shadow file support.  Hallelujah.
- Disabled/commented out most commands, ready for the big transition
  to the new parsing style.
- Obsoleted Subst.{h,cpp}.

src/CMakeLists.txt

@@ -25,7 +25,6 @@ set (task_SRCS API.cpp API.h
                Permission.cpp Permission.h
                Record.cpp Record.h
                Sequence.cpp Sequence.h
-               Subst.cpp Subst.h
                TDB.cpp TDB.h
                TDB2.cpp TDB2.h
                Task.cpp Task.h