binary helpers: add "invalid" enum item, improve readability

binary helpers: add "invalid" enum item, improve readability

The "binary value" helpers would not follow the sigrok project's style
of only using non-zero values for valid enum items (a zero value is
assumed to always represent something "inactive" or "invalid").

Introduce the "invalid" item in the enum declaration to fixup the style
issue. Rephrase the bv_get_value() routine's length specs to improve
readability, match numbers in length specs to the numbers in conversion
routine references.