rpcs:
+---x get-system-uptime
+--ro output
+--ro show-system-uptime
| +--ro days? common-def:time-interval-day
| +--ro hours? common-def:time-interval-hour
| +--ro minutes? common-def:time-interval-minute
| +--ro seconds? common-def:time-interval-sec
+--ro cmd-error? string