Use sane default sendmail options

mikel/mail uses some standard sendmail options, which we should also
use.
They can always be overridden in configuration.yml or appropriate
environment variables.

see:
https://github.com/mikel/mail/blob/1caf1b83c112ac7df2c7d95c3aa4d5604857235c/lib/mail/network/delivery_methods/sendmail.rb#L43
Merge pull request #1677 from opf/fix/sendmail_default_options

Use sane default sendmail options

I totally know what I am doing here...