cgroup: Add ManagedOOMMemoryPressureDurationSec= override setting for units

This will allow units (scopes/slices/services) to override the default
systemd-oomd setting DefaultMemoryPressureDurationSec=.

The semantics of ManagedOOMMemoryPressureDurationSec= are:
- If >= 1 second, overrides DefaultMemoryPressureDurationSec= from oomd.conf
- If is empty, uses DefaultMemoryPressureDurationSec= from oomd.conf
- Ignored if ManagedOOMMemoryPressure= is not "kill"
- Disallowed if < 1 second

Note the corresponding dbus property is DefaultMemoryPressureDurationUSec
which is in microseconds. This is consistent with other time-based
dbus properties.

docs/TRANSIENT-SETTINGS.md

@@ -281,6 +281,7 @@ All cgroup/resource control settings are available for transient units
 ✓ ManagedOOMSwap=
 ✓ ManagedOOMMemoryPressure=
 ✓ ManagedOOMMemoryPressureLimit=
+✓ ManagedOOMMemoryPressureDurationSec=
 ✓ ManagedOOMPreference=
 ✓ CoredumpReceive=
 ```