smaller journal fixes (#3124)

* sd-journal: detect earlier if we try to read an object from an invalid offset

Specifically, detect early if we try to read from offset 0, i.e. are using
uninitialized offset data.

* journal: when dumping journal contents, react nicer to lines we can't read

If journal files are not cleanly closed it might happen that intermediaery
journal entries cannot be read. Handle this nicely, skip over the unreadable
entries, and log a debug message about it; after all we generally follow the
logic that we try to make the best of corrupted files.

* journal-file: always generate the same error when encountering corrupted files

Let's make sure EBADMSG is the one error we throw when we encounter corrupted
data, so that we can neatly test for it.

* journal-file: when iterating through a partly corruped journal file, treat error like EOF

When we linearly iterate through a corrupted journal file, and we encounter a
read error, don't consider this fatal, but merely as EOF condition (and log
about it).

* journal-file: make seeking in corrupted files work

Previously, when we used a bisection table for seeking through a corrupted
file, and the end of the bisection table was corrupted we'd most likely fail
the entire seek operation. Improve the situation: if we encounter invalid
entries in a bisection table, linearly go backwards until we find a working
entry again.

* man: elaborate on the automatic systemd-journald.socket service dependencies

Fixes: #1603

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