meson: Introduce initconfdir option

meson: Introduce initconfdir option

Right now we expect the configuration files for init scripts
to live in /etc/sysconfig, but that location is only used by
RHEL- and SUSE-derived distros.

This means that packagers for other distros have to patch
things as part of the build process, while people building
from source will get wonky integration.

This new option will provide a convenient way to override
the default location at build time that is usable by distro
packagers and people building from source alike.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>