gw: fix config and systemd example generation

[ralphlange]

The generation of example configuration and systemd files was a bit messed up:

• systemd service was starting pvagw with itself (instead of the config file)
• Parsing the instance name from the command line was broken for the systemd file generation
• Printing to "-" was suggesting to save the config file as systemd file

With this change...

• pvagw generates a systemd service template for a filename *@.service and a systemd service instance for *@instance.service.
• Documentation clarifies that the shown command is creating a systemd service instance file.

[mdavidsaver]

Would you be so kind as to extend my clearly inadequacy test case?

https://github.com/mdavidsaver/p4p/blob/936b12a978d68ce4153c83c73cc313e74ec0473d/src/p4p/test/test_gw.py#L55

[ralphlange]

Oops.
I completely missed that. Sure, of course.

[ralphlange]

Duh.
"Like tempfile.NamedTemporaryFile..." doesn't include being able to use the same keyword args. You assume open() args instead.
How do you want me to resolve this?

[ralphlange]

All's well that ends well...

[ralphlange]

Ping @mdavidsaver ...

[ralphlange]

Ping-ping @mdavidsaver ...

[ralphlange]

Merci!