sasl: Fix sending of long authentication information
sasl: Fix sending of long authentication information

The specification requires AUTHENTICATE information to be split into
chunks of size 400. This commit implements the necessary chunking.

Fixes: https://github.com/znc/znc/issues/942
Signed-off-by: Uli Schlachter <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

znc/znc,znc/znc,znc/znc,znc/znc,znc/znc,znc/znc