Fixes #64

Relates to #53

Refers to #55

Since the executable will be located in the service base directory it
makes most sense to check in the `var` subdir for the config by default.

Fixes #58

Fixes #51

Refers to #53

Fixes #56

Fixes #53

Fixes #55

Fixes #54

Refers to #50

That's the earliest place where setting up the mail logger is possible /
makes sense, so do it right then. Additionally, the check for SmtpHost
is redundant since it also done in the method called, so no need for it.

Refers to #50

Refers to #35

This reverts commit 3f7fd9dc.

As intended and announced in the previous commit message.

Relates to #20

NOTE: this is known to fail for permission reasons, the commit is in
fact just to save the code and serve as an example (and to be reverted
right away).

Relates to #20

This way the service should now finally receive the system shutdown
events and be able to properly shut itself down.

Fixes #45

Fixes #41

Fixes #47