sysupdate: Add a stub varlink interface and implement CheckNew (#42422)

This puts the scaffolding in place for a varlink interface, but so far
it only adds a `io.systemd.Sysupdate.CheckNew()` method. Varlinkifying
the other verbs on `systemd-sysupdate` will happen in follow-up PRs, but
I thought I’d try and land this one early to:
 * Get review of the overall varlinkification scaffolding
* Lower the chance of big merge conflicts with others’ work by getting
the more invasive changes out of the way
* Get the scaffolding in place so others can start to build on it if
they wish (although I am currently working on porting the other existing
verbs)

It rearranges how the `Context` struct is allocated so that it’ll be
easier to add per-method/verb context structs which contain it in
future. It also changes all the `sysupdate.c` code to use arguments from
`Context` rather than `arg_*` globals, allowing them to be specified as
varlink parameters in future.

It also moves the existing `systemd-sysupdate.{timer,service}` units
(which periodically run `systemd-sysupdate update`) to
`systemd-sysupdate-update.{timer,service}` to clear space for a
`systemd-sysupdate@.service` and `systemd-sysupdate.socket` to act as a
varlink entry point.

/cc @AdrianVovk