base64: make decoder handle decoded data space constraints

base64: make decoder handle decoded data space constraints

So far, it was the job of caller to send the bae64 decoder a perfect
block of data and take care of the destination buffer (decoded data)
size. Now, make it the decoder's job to take care of any space
constraints that the destination buffer may have and return accordingly.

Also, handle space characters in base64 encoded data as per RFC 2045.

Update MIME parser accordingly to handle the base64 data.

Ticket: 5315