hostnamed: make chassis type configurable via /etc/machine-info

hostnamed: make chassis type configurable via /etc/machine-info

For many usecases it is useful to store the chassis type somewhere, and
/etc/machine-info sounds like a good place. Ideally we could always
detect the chassis type from firmware, but frequently that's not
available and in many embedded devices probably entirely unrealistic.

This patch adds a configurable setting CHASSIS= to /etc/machine-info and
exposes this via hostnamectl/hostnamed. hostnamed will guess the chassis
type from DMI if nothing is set explicitly. I also added support for
detecting it from ACPI, which should be more useful as ACPI 5.0 actually
knows a "tablet" chassis type, which neither DMI nor previous ACPI
versions knew.

This also enables DMI-based and ACPI-based detection for non-x86 systems
as ACPI is apparently coming to ARM platforms soon.

I tried to minimize the vocabulary of chassis types understood and
added: desktop, laptop, server, tablet, handset. This is much less than
either APCI or DMI know. If we need more types later on we can easily
add them.