This introduces a bunch of facilities:
1. The factory-reset.target unit that requests a factory reset is now
complemented by factory-reset-now.target that executes it at next
boot.
2. This latter is added to the initial transaction via the new trivial
systemd-factory-reset-generator.
3. A tool systemd-factory-reset has been added to query, request,
cancel, complete factory reset operations (via EFI variables). Two of
these are wrapped into units that are plugged into
factory-reset.target and factory-reset-now.target respectively. The
tool also provides a simple Varlink API.
This should make things a lot cleaner, and both be useful as explicit
implementation on UEFI, and as template + hookpoints for alternative
implementations on non-UEFI.
