bin/base64: add command line options for decoding

[aklomp]

Add the following command line options to the bin/base64 demo utility that affect decoding:

• --no-strip-newlines|-n: turn off the recently introduced check for newlines in the input. This speeds up decoding for any inputs that are guaranteed not to have newlines in them.
• --ignore-garbage|-i: taken from GNU base64, this option ignores any characters in the input which are not part of the base64 alphabet. This turns out to be fairly easy to implement on top of the newline detection code, so might as well add it.