Three fixes:
1. ExecHosts.UnmarshalJSON: JSON null (sent when no allocation exists)
was treated as a non-"-" string because json.Unmarshal(null, &str)
returns nil with str="". This created a phantom HostSlot{NumSlots:1,
HostName:""} producing "<1*>" for every PEND job. Now null and ""
are both treated as empty hosts.
2. page_print_jobs.go: added !startTime.IsZero() guard so a zero
timestamp ("0001-01-01T00:00:00Z") never produces a start line
even if the status check is bypassed.
3. Added PENDING_REASON field to JobFieldsMap so it is requested from
the API when -l is passed, and rendered in printJobsVerboseOutput
for PEND/PSUSP jobs before the start-time section.
