bug#56799: (gnu services configuration) usage of *unspecified* is problematic

[Maxim Cournoyer]

Hi Attila,

[...]

> i need to run now, and i'll be offline for a week or two. i can't look
> the example in depth now, but my gut instinct says that it's a bug if
> *unspecified* reaches any GExp machinery.

I don't think it's reasonable to burden users with normalizing their
G-exp input data, where *unspecified* may appear in nested data
structures (such as used by the jami-service-type: jami-accounts has
maybe fields end is used as a nested data type to jami-configuration).

I think v3 of this patch enables us to continue with our current ways
and is a non-invasive change, so I'll merge it soon if there are no
objections.

Thanks,

Maxim