journal: rename special journal field _SYSTEM_CONTEXT= → _RUNTIME_SCOPE=

journal: rename special journal field _SYSTEM_CONTEXT= → _RUNTIME_SCOPE=

Previously the field "_SYSTEM_CONTEXT" knew he values "initrd" + "main". Let's change
this to "_RUNTIME_SCOPE" and "initrd" + "system".

Why? The sysext logic has a very similar concept of "scopes", declaring
whether a sysext image is intended for the initrd or the main system.
Let's thus use the same naming for both.

sysext's extension-release files hence know SYSEXT_SCOPE=initrd|system,
and the journal messages know _RUNTIME_SCOPE=initrd|system, which makes
this reasonably systematic.

Follow-up for: cae8edd93ca2ef90c41cb9b6322b6908d12947b5

(This is not an API break, since no version with this commit has ever
been released.)