Bug 26363: Provide a systemd unit file for koha-worker for source installs

Bug 26363: Provide a systemd unit file for koha-worker for source installs

This patch adds a systemd unit file which gets rewritten according to
the configuration specified by Makefile.PL. This can be linked or
copied to systemd directories for running a systemd service for
background_jobs_worker.pl

Test plan:
1. perl Makefile.PL
1a. Choose "single" installation
1b. Choose "/opt/koha" for a file path
2. make
3. vi blib/KOHA_CONF_DIR/koha-worker.service
4. Verify that PERL5LIB, KOHA_CONF, ExecStart, and SyslogIdentifier
all appear to be correctly filled in

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>