systemd.unit support for filenames with hyphens

[nharward]

When using /etc/kmonad/*.kbd files with hyphens in the name systemd
will escape them to slashes when using %I. This commit changes to use
%i instead which does not escape the name, resulting in the correct
filename passed to kmonad.

It's important to use %i for the filename so that multiple instances
of kmonad can run for different keyboards at the same time, say for a
laptop and external keyboard. The DefaultInstance=config in line 11
will use a default value of config if no other value is given.

Examples:

`systemctl start kmonad@foo`     # Runs `kmonad /etc/kmonad/foo.kbd`
`systemctl start kmonad@foo-bar` # Runs `kmonad /etc/kmonad/foo-bar.kbd`
`systemctl start kmonad@`        # Runs `kmonad /etc/kmonad/config.kbd`

[nharward]

Looks like the failed workflow step was GHC installation on Windows. Is it possible to re-trigger this flow to see if it was intermittent, or push through regardless? This PR is only for a contrib file related to systemd and does not affect kmonad core code.

cc @slotThe

[slotThe]

Oh, sorry, I must have forgotten about this. AFAIR the workflow is due to some upstream issue; should go away by itself

Thanks!