MI: switch from libsdbus to libdbus

MI: switch from libsdbus to libdbus

libsystemd is not generally available as a static library, and we would
like to produce a static nvme-cli. This change switches to libdbus
instead.

This requires slightly more boilerplate code for the dbus
marshalling/unmarshalling as part of the MCTP endpoint scan, but means
we have a simpler upstream lib dependency.

Fixes: https://github.com/linux-nvme/nvme-cli/issues/1734
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>