adapt journalctl invocation to buster

adapt journalctl invocation to buster

With Debian Buster the behavior of `journalctl` has changed when it finds
no entries for a given selection:
* The exit code was 0 in stretch, but is 1 in buster
* The output changed slightly - a header got added

Since PMG::Utils::scan_journal_for_rbl_rejects uses journalctl for reading the
IPs blocked by postscreen it needs to adapt for the new behavior (otherwise
run_command dies because of the exit code 1)

The patch addresses the problem by using the json-output of `journalctl`, which
still exits with 0 if no entries are present . Additionally the json-output
adds the current cursor to the output by default, removing the need to
explicitly scanning for it.(the exit code of 1 was due to '--show-cursor'
without a single result line).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>