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