This patch eliminates a line length limit in pop3resp.  It changes
pop3resp to use Brdstr rather than Brdline.  The patched pop3resp
still takes care of memory management for its callers: A line is freed
when the next line is read.

The limit causes upas/fs to blow up with "unexpected eof" and
"unexpected end of conversation" when unreasonable messages exist on a
POP server.