fix(journald source): Re-fix journald cursor handling and libsystemd name (#1202)

* Fix systemd dynamic library name

The correct name for the systemd library is `libsystemd.so.0` and not
`libsystemd.so`.

Signed-off-by: Bruce Guenter <bruce@untroubled.org>

* Rework journald read state logic

The two read states (saw-record vs at-end) are actually independent and
not mutually exclusive, so representing them as an enum leads to flow
control errors. This led to breaking a previous fix for issue #1081
(journald source fails to retrieve checkpoint).

Signed-off-by: Bruce Guenter <bruce@untroubled.org>

* Break up the overly-long `JournaldServer::run` function

Signed-off-by: Bruce Guenter <bruce@untroubled.org>