feed forwarded mails into proxmox_notify

feed forwarded mails into proxmox_notify

This allows us to send notifications for events from daemons that are
not under our control, e.g. zed, smartd, cron. etc...

For mail-based notification targets (sendmail, soon smtp) the mail is
forwarded as is, including all headers.
All other target types will try to parse the email to extra subject
and text body.

On PBS, where proxmox-notify is not yet fully integrated,
we simply add a default target/matcher to an empty config. That way
the behavior should be unchanged - mails will be forwarded to
root@pam.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>