Add a CFLG_OBSOLETE flag, and handle it at the confvar layer

Follow up to legacy/trac#32295 (moved) for 0.4.3.

nickm says:

My suggestion would be to change how OBSOLETE is implemented: it doesn't really make sense as a type. Instead, there should be an "empty" type for fields that can't be read or written, and a CFLG_OBSOLETE flag that causes these warnings. The CFLG_OBSOLETE flag should be handled at the confvar layer, I think.

This is a bug caused by sponsor 31, so it's a must-have.