Change menu support to allow programmed menus

Menu fields currenly only support a fixed set of strings with a 1-1 mapping to an index value. For several feature enhancements it would be useful to accept alias strings, and even the ability to add menu choices at run-time. The best way to implement these could be to allow menu support to register itself with the database (mset) and to provide routines for conversion to/from string values. For backwards compatibility (with RSRV) there probably needs to be an array of strings giving the canonical name for each index value, but the API could allow menu users to register for callbacks when the list of choices changes.

Record enum fields could possibly be implemented through the same API, and maybe even the DTYP field (DBF_DEVICE).