Downloading attachments is fixed. Now we do not buffer the entire attachment

Downloading attachments is fixed. Now we do not buffer the entire attachment
before echo it to stdout. with fread we detect how long the base64 encoded
liones are so we can do base64 decoding on the fly.
The advantage of using fread instead of fgets is that the buffersize can be
used so we reduce the number of echo calls (fread doesn't stop at \n).

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5137 7612ce4b-ef26-0410-bec9-ea0150e637f0