Only define `@` and `_NARG` when they have values

[Rangi42]

Currently using @ outside a section will error "PC has no value outside a section" and evaluate to 0. Using _NARG outside a macro will error "_NARG does not make sense outside of a macro" and evaluate to 0.

These special errors could be avoided if the symbols were simply not defined when they don't have sensible values. That would also let a macro detect whether it's in a section by doing if DEF(@).

[Rangi42]

It may be simpler to leave their numeric value callbacks always in place, and modify how DEF works to potentially report them as undefined. Because for most symbols, even if they're undefined, things like db SYMBOL will still work, leaving the actual value to be determined later on. But dw @ outside a section, or db _NARG outside a macro, can/should never work.