Feed 'journalctl' stderr into log stream

Make errors from `journalctl` itself filterable by integrating its stdout into
stream of collected messages.

Useful for working around corrupted journal files.

Switch from egrep to 'grep -E'

filters: Update journalctl filter to match new footer

Use journal cursor instead of timestamp

Fix inaccuracy regarding invocation state: using the invocation timestamp
obtained from `date` is too coarse for high-frequency journal entries and might
lead to entries getting lost.

Fix by dumping the journal cursor on each invocation and re-use it as reference
for the next run. This should make journalcheck waterpoof regarding dump
integrity.