Optional features allow distros to define sets of transfers that can
be enabled or disabled by the system administrator. This is useful for
situations where a distro may want to ship some resources version-locked
to the core OS, but many people have no need for the resource, such as:
development tools/compilers, drivers for specialized hardware, language
packs, etc
We also rename sysupdate.d/*.conf -> sysupdate.d/*.transfer, because
now there are more than one type of definition in sysupdate.d/. For
backwards compat, we still load *.conf files as long as no *.transfer
files are found and the *.conf files don't try to declare themselves
as part of any features
Fixes https://github.com/systemd/systemd/issues/33343
Fixes https://github.com/systemd/systemd/issues/33344
