journalctl: Periodically call sd_journal_process in journalctl

If `journalctl` take a long time to process messages, and during that
time journal file rotation occurs, a `journalctl` client will keep
those rotated files open until it calls `sd_journal_process()`, which
typically happens as a result of calling `sd_journal_wait()` below in
the "following" case.  By periodically calling `sd_journal_process()`
during the processing loop we shrink the window of time a client
instance has open file descriptors for rotated (deleted) journal
files.

(Lennart: slightly reworked version, that dropped some of the commenting
which was solved otherwise)

(cherry picked from commit ec316d19)