switch from libsystemd's dbus to dbus-1

switch from libsystemd's dbus to dbus-1

This is purely so that we can do static linking.  Linking against
libsystemd makes that a challenge because while it's perfectly simple
to do, distros tend not to provide a libsystemd.a.

Tools that want to (a) link against liblxc and (b) have a statically
linked binary to bind into a minimal container are ill served by
this.  So link against libdbus-1.

.github/workflows/build.yml: switch to dbus-1.
src/lxc/cgroups/cgfsng.c: replace the unpriv_systemd_create_scope(),
   start_scope, and enter_scope() systemd code with dbus-1 code.
src/tests/oss-fuzz.sh: update from libsystemd-dev to libdbus-1-dev
src/tests/oss-fuzz.sh: disable dbus
.github/workflows/*: update from libsystemd-dev to libdbus-1-dev
meson.build and meson_options.txt: switch from sd_bus to dbus
lxc.spec.in: add dbus-1 to BuildRequires

Signed-off-by: Serge Hallyn <serge@hallyn.com>
Changelog: 03/13: use custom iter type so we can cleanup more easily...
Changelog: 03/13: initialize each dbus_iter to { 0 } as mihalicyn suggested.