nntp: small speed up for multi-line responses

nntp: small speed up for multi-line responses

Using a non-zero-length separator for `join' requires extra work
inside Perl.  We can shove the cost of appending "\r\n" into the
`map' loop, instead.  This speeds up the `join' operation.

The "deferred" log entry for a "LISTGROUP org.kernel.vger.linux-kernel"
command (with nearly 3.8 million messages) goes from ~3.96s to 3.86s
on my workstation.