journal-remote: use source's boot-id

systemd-journal-remote always wrote the boot-id of the device it was running on
to the header of its journal files. When the source had a different boot-id
(because it was generated on a different boot, or a different device), the
boot-ids in the file were inconsistent. The _BOOT_ID field was that of the
source, but the journal file header and each entry object header were that of
the device systemd-journal-remote ran on. This breaks journalctl --list-boots
on any of these files.

Set the boot-id in the header to be that of the source. This also fixes the
entry object headers.