Features:
* bootctl: add "gc" verb that loads all type #1 .conf files, and then removes
- all files from the set of files matching the entry token that are not
- referenced by any.
+ all files from the set of files from the ESP/XBOOTLDR matching the entry
+ token that are not referenced by any. Then, change kernel-install to use only
+ this to remove auxiliary files, and never remove them explicitly. Benefit:
+ resources such as initrds/kernels/dtb can be shared between entries.
* networkd/udevd: add a way to define additional .link, .network, .netdev files
via the credentials logic.