Split EnumIntOption from StringOption

Split EnumIntOption from StringOption

Remove storeEnumIndex() from StringOption, and add EnumOption that
supports the same input values as StringOption with enumValue(), but
stores the result in an integer variable.

This removes complexity from StringOption(), where managing all possible
combinations of store(), storeEnumIndex(), and their defaults reasonably
was quite complicated.

Increase use of enum types instead of string comparisons.

Change-Id: I2f1bca204af83dc404b6f20842119d1b7d4a27ad