Fixed up $A multi-command-line-arg parsing.

Added StringExtractor::GetHexByteStringFixedLength(str, nibble_length).
This collects exactly the given number of nibbles (i.e. 2x the expected
return string size).  It returns the CString length actually parsed
if the input ended before the given nibbles could be consumed.  It will
not consume more than the given number of nibbles, though.