mdadm: allow sending mail when using opensmtpd

OpenSMTPD does not require the setuid bit for its `sendmail`. This works
around it by wrapping the called `sendmail` so that the wrapper falls
back on either the setuid `sendmail` or the non-setuid `sendmail`
depending on what's available.

The solution of relying on `$PATH` to be set is unfortunately
unreliable, as `mdadm --monitor` will likely be executed from a
`systemd` unit, that runs with a clean `$PATH`.