zcash-inspect: Test signatures in pre-v5 transactions against all known consensus branch IDs

[str4d]

Currently pre-v5 transactions are loaded with the Sprout consensus branch ID, which leads to signature verification problems. We don't know what consensus branch ID was used just by looking at the format, so for informational purposes we should attempt to figure it out (and if we can't, then print a warning that signatures could not be validated).