Update user accounting database once time is synced

[citrus-it]

(PST within the zones notwithstanding, see
oxidecomputer/helios-omicron-brand#4)

This change rewrites the user accounting databases for all running zones
(including the GZ) once time is synchronised. It uses the
/usr/platform/oxide/bin/tmpx command which is available in very recent
stlouis. If that's missing, it will just carry on regardless, leaving the
system uptime showing 13k days as before.

{"msg":"Timesync for [fd00:1122:3344:101::1]:12345 TimeSync { sync: false, skew: 0.0, correction: 0.0 }","v":0,"name":"SledAgent","level":30,"time":"1986-12-28T00:15:45.669141687Z","hostname":"gimlet-sn06","pid":101256,"component":"RSS"}
{"msg":"Time is not yet synchronized","v":0,"name":"SledAgent","level":40,"time":"1986-12-28T00:15:45.669154267Z","hostname":"gimlet-sn06","pid":101256,"component":"RSS","error":"\"Time is synchronized on 0/1 sleds\""}
...
gimlet-sn06 # date; uptime
Sun Dec 28 00:26:49 UTC 1986
00:26:49    up 1 min(s),  2 users,  load average: 1.31, 1.54, 0.77
gimlet-sn06 # zlogin oxz_internal_dns 'date; uptime'
Sat Dec 27 16:27:07 PST 1986
16:27:07    up 1 min(s),  0 users,  load average: 0.00, 0.00, 0.00
...
{"msg":"Timesync for [fd00:1122:3344:101::1]:12345 TimeSync { sync: true, skew: 117.186, correction: 2.207e-5 }","v":0,"name":"SledAgent","level":30,"time":"2023-04-11T22:34:20.543455275Z","hostname":"gimlet-sn06","pid":101256,"component":"RSS"}
...
gimlet-sn06 # date; uptime
Tue Apr 11 23:28:19 UTC 2023
23:28:19    up 1 min(s),  2 users,  load average: 4.75, 2.14, 1.10
gimlet-sn06 # zlogin oxz_internal_dns 'date; uptime'
Tue Apr 11 16:28:23 PDT 2023
16:28:23    up 1 min(s),  0 users,  load average: 0.00, 0.00, 0.00