BUG/MINOR: cli: cli_snd_buf: preserve \r\n for payload lines

BUG/MINOR: cli: cli_snd_buf: preserve \r\n for payload lines

cli_snd_buf() analyzez input line by line. Before this patch it has always
scanned a given line for the presence of '\r' followed by '\n'.

This is only needed for strings, that contain the commands itself like
"show ssl cert\n", "set ssl cert test.pem <<\n".

In case of strings, which contain the command's payload, like
"-----BEGIN CERTIFICATE-----\r\n", '\r\n' should be preserved
as is.

This patch fixes the GitHub issue #2818.

This patch should be backported in v3.1 and in v3.0.