sd-bus: properly support passing memfds around with offset/size parameters
sd-bus: get rid of PID starttime concept

As kdbus no longer exports this, remove all traces from sd-bus too
update TODO
run: introduce timer support option

Support timer options --on-active=, --on-boot=, --on-startup=,
--on-unit-active=, --on-unit-inactive=, --on-calendar=. Each options
corresponding with OnActiveSec=, OnBootSec=, OnStartupSec=,
OnUnitActiveSec=, OnUnitInactiveSec=, OnCalendar= of timer
respectively. And OnCalendar= and WakeSystem= supported by
--timer-property= option like --property= of systemd-run.

And if --unit= option and timer options are specified the command can
be omitted. In this case, systemd-run assumes the target service is
already loaded. And just try to generate transient timer unit only.
bus-proxy: cloning smack label

When dbus client connects to systemd-bus-proxyd through
Unix domain socket proxy takes client's smack label and sets for itself.

It is done before and independent of dropping privileges.

The reason of such soluton is fact that tests of access rights
performed by lsm may take place inside kernel, not only
in userspace of recipient of message.

The bus-proxyd needs CAP_MAC_ADMIN to manipulate its label.

In case of systemd running in system mode, CAP_MAC_ADMIN
should be added to CapabilityBoundingSet in service file of bus-proxyd.

In case of systemd running in user mode ('systemd --user')
it can be achieved by addition
Capabilities=cap_mac_admin=i and SecureBits=keep-caps
to user@.service file
and setting cap_mac_admin+ei on bus-proxyd binary.
unit: update unit dropin paths and time when dropin file is written.

If a unit is set property by "systemctl set-property", a new dropin
file is generated. But the unit's dropin_paths and dropin_mtime are
not updated. So the unit is shown as need daemon reload.
Update unit dropin_paths and dropin_mtime also when dropin file is
written.