Remove support for the Binary Content extension [RFC3516].

Remove support for the Binary Content extension [RFC3516].

“If the server does not know how to decode the section's CTE, it MUST
fail the request and issue a "NO" response that contains the
"UNKNOWN-CTE" extended response code.” — [RFC3516 section 4.3]

Unfortunately the client doesn't know which message couldn't be decoded,
so it can't fallback and use BODY instead.

This made ‘use-binary=NO’ pretty much mandatory.  Hence we remove
support for BINARY [RFC3516].  Instead, we increase the thresold for
when to add Zlib full flush points from 4096 to the buffer size (32768).