add_enum

add a new enum type
     idx - serial number of the new enum.
           If another enum with the same serial number
           exists, then all enums with serial
           numbers >= the specified idx get their
           serial numbers incremented (in other words,
           the new enum is put in the middle of the list
           of enums).
           If idx >= get_enum_qty() or idx == -1
           then the new enum is created at the end of
           the list of enums.
     name - name of the enum.
     flag - flags for representation of numeric constants
            in the definition of enum.
returns: id of new enum or -1.

long add_enum(long idx, string name, long flag);