Fix smail so it passes job arrays to seff correctly

Fixes smail to handle change in slurmctld/agent.c#mail_job_info()
output introduced in commit 5f1cc8a8.
seff has incomplete job array support, but if one happens to come
through smail, at least let it print out what it can (i.e. the last
array task in the job array).

Bug 6536