Fix libxo output from uptime command

the libxo output for uptime returned multiple 'uptime' keys, one each for number of days, hours, and minutes of uptime.
This is invalid JSON.
This patch makes the output the raw number of seconds, as well as adding keys for the individual unit values
A string of the original output from the plain-text uptime command is also added

Differential Revision: https://reviews.freebsd.org/D2063
Reviewed by: jmg
Approved by: marcel
Sponsored by: ScaleEngine Inc.