sudo: add sendmailPath option (defaults to /var/setuid-wrappers/sendmail)

It currently seems impossible to make sudo send emails. No matter how much
debugging I enable, sudo continues to be silent about sendmail. I tried setting
"Defaults mailerpath=/var/setuid-wrappers/sendmail" but that too was completely
ignored (except for logging that the variable mailerpath exists with the value
I assigned to it...).

This enables sendmail support and sets a default value that works on NixOS.