journal-file: don't update boot_id in journal header on open

journal-file: don't update boot_id in journal header on open

The header of the journal file contains a boot ID field that is
currently updated whenever we open the journal file. This is not ideal:
pretty often we want to archive a journal file, and need to open it for
that. Archiving a foreign journal file should not mark it as ours, it
should just change the status flag in the file header.

The boot ID in the header is aleady rewritten whenever we write a
journal entry to the file anyway, hence all this patch effectively does
is slightly "delay" when the boot ID in the header is updated: instead
of immediately on open it is updated on the first entry that is written.

Net effect: archived journal files don't all look like they were written
to on a boot newer then they actually were

And more importantly: the "tail_entry_monotonic" field suddenly becomes
useful, since we know which boot it belongs to. Generally, monotonic
timestamps without boot ID information are useless, and this fixes it.

A new (compatible) header flag marks file where the boot_id can be
understood this way. This can be used by code that wants to make use of
the "tail_entry_monotonic" field to ensure it actually can do so safely.

This also renames the structure definition in journal-def accordingly,
to indicate we now follow the stricter semantics for it.