dma: limit lines to 998 characters

Per RFC2822 the maximum transmitted line length is "998 characters...
excluding the CRLF." In a file the maximum is 999 with the \n included.

Previously mail containing a line with exactly 999 characters would
bounce.

PR: 208261
Reported by: Helge Oldach
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 1a0dde338df8b493d74dcb2f7bbaaa6c02cab371)