Define a dedicated `vmvar_t` type for values stored in variable pages
and propagate it through interfaces that expose those values. Keep the
type as int for now so this commit does not change runtime behavior.
This separates VM storage from ordinary int buffers and makes a later
conversion to 16-bit storage mechanically checkable.
This replaces v_str* functions with new svar_* functions.
The new svar_* functions use 1-based string variable index, so that
callers don't have to decrement string numbers from SCO.
kichikuou