Follow-up for #26902 and #26971
Let's always calculate the next restart interval
since that's more useful.
For that, we add 1 to s->n_restarts unconditionally,
and change RestartUSecCurrent property to RestartUSecNext.
Unit names can be 255 characters long, not 256.
We first say "name prefix" and then continue with "unit prefix".
Confusing. Couldn't figure out which term is better hence settled on
"unit name prefix".
sysext DDI cannot carry an os-release file, but have to carry
an extension-release file. But so far, this was only used to
match the sysext DDI with the base DDI/rootdir. It is also
useful to describe the sysext DDI itself, just like we do in
os-release.
So document that the same fields used in os-release can also
be added to an extension-release, with the 'SYSEXT_' prefix,
and in that case they are understood to define the sysext DDI
itself, rather than for matching purposes.
interval between restarts
RestartSteps= accepts a positive integer as the number of steps
to take to increase the interval between auto-restarts from
RestartSec= to RestartSecMax=, or 0 to disable it.
Closes#6129
Define $KERNEL_INSTALL_UKI_GENERATOR in case one wants it to be different from $KERNEL_INSTALL_INITRD_GENERATOR. This can be useful if one wants to use mkinitcpio / Dracut to generate the initrd, but without creating the UKI so this can be left for e.g. ukify or something else. Right now these initrd generators will read /etc/kernel/install.conf and generate the UKI
