improve drain message: "unkillable processes for job fNiY3EqzUPq"

[garlick]

Problem: job-exec drains nodes with the message in the subject line. Sys admins would like the message to indicate a) that it's user processes that are part of a job that are unkillable, as opposed to housekeeping or a flux process, and b) if possible, indicate the hostlist that had unkillable processes.

[grondo]

if possible, indicate the hostlist that had unkillable processes.

I think only the hosts that had unkillable processes are drained, and they should be drained at the same time, so the default output of flux resource drain should by default show the affected hosts?

[garlick]

@kkier might want to weigh in here.