util-base64: strict mode - all characters must be valid

util-base64: strict mode - all characters must be valid

Introduce a strict mode to base64 decode. If strict,
the function will fail when invalid input data is seen.
If not strict, what has been decoded will be returned.

This is in support of adding a Snort compatible base64_decode
rule option that uses whatever data can be decoded as a length
of data to decode is optional.