shared/bootspec: don't warn for new `loader.conf` options and correctly parse new...

Commit e2a3d562189c413de3262ec47cdc1e1b0b13d78b added the `uki` option
to sd-boot, and 1e9c9773b994f2f703a5aa5ba80961e90be3a892 added
`profile`, but because these were not added in src/shared/bootspec,
bootctl still shows warnings like `Unknown line 'uki', ignoring.` when
parsing the config. This PR allows parsing and displaying them correctly
in `bootctl` output. It also stops it from printing a warning for any of
the new `loader.conf` options (`log-level`, `reboot-on-error`, etc.).
Note that `uki-url` is still not handled as I can't easily test it.