logger: concatenate multiple lines of MESSAGE into a single field.

logger: concatenate multiple lines of MESSAGE into a single field.

this is deemed a useful special case since journalctl will only show
either the first or last element of the message array if the field
appears multiple times.

Based on patch from: Kjetil Torgrim Homme <kjetil.homme@redpill-linpro.com>
https://github.com/karelzak/util-linux/pull/743

Addresses: https://github.com/karelzak/util-linux/issues/742
Signed-off-by: Karel Zak <kzak@redhat.com>