It appears that several versions of sendmail require that the mail is sent to them with
LF line endings instead of CRLF endings - which of course they will then convert back
to CRLF line endings to comply with the SMTP standard.

This PR adds another setting SENDMAIL_CONVERT_CRLF which will pass the message writer
through a filter. This will filter out and convert CRLFs to LFs before writing them
out to sendmail.

Fix go-gitea#18024

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>

Emails are relatively small so just buffer and replace instead of filtering
\r\n through the writer.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>