mkosi: generate default UTF-8 locale for Arch Linux

[johnlinp]

Fixes: #186

[poettering]

grrr. A pity that arch doesn't get this right out-of-the-box

[poettering]

hmm, this approach leaves locale-gen.service around in the image. I'd prefer if we didn't. Can't we instead just invoke localegen through nspawn, after running the package manager?

[johnlinp]

@poettering Thank you for the comments. I will work on it.

[johnlinp]

In the new patch, I use run_workspace_command() to execute locale-gen instead of creating a service unit. Please take a look at it. Thanks.