Rework support for Debian-specific locale, keyboard and timezone conf files.

This splits existing patches into separate patches for each indepenent feature.

Also drops "timedated-don-t-rely-on-usr-being-mounted-in-the-ini.patch", as its
design is flawed: timedated does not care about the content of /etc/localtime,
only the symlink value. To fix the actuall problem, that timezoned needs zoneinfo
data which is located in /usr, add RequiresMountsFor=usr/share/zoneinfo to the
service file.
Rework the semantics of restart job deadlock avoidance during early boot and shutdown.
Replace one sysv-generator patch with the version I submitted upstream.