Mail: introduce new INI for line ending control #19188

Description

There has been few reports about clients that require LF for sandmail messages. It seems that some clients have such expectation and people need to add dos2unix to sendmail_path. One of the suggested solution is here: #8086 (comment) .

It would be useful to introduce a new mode that would give a better control. That could be called mail.cr_lf_mode and it could allow values crlf (current default), lf (new option doing what dos2unix does), mixed (that would be what mail.mixed_lf_and_crlf = on), os (crlf on Win, otherwise lf). It means there would be an additional allowing conversion to LF.

I'm not sure about the default yet but initially it would be better to keep it as crlf to not cuase further confusion with another break.