flux-start: only a single local session is supported

[grondo]

For running tests in parallel it will be nice if flux-start allowed multiple local comms sessions
to start at the same time. Currently I get

$ ./flux start --size=2
cmbd: cmbd is already running in /var/tmp/grondo/flux-0-1, pid 84630
cmbd: cmbd is already running in /var/tmp/grondo/flux-0-0, pid 84629
flux-start: 0 (pid 84679) exited with rc=1
flux-start: 1 (pid 84680) exited with rc=1

I'm guessing this will probably be a trivial fix, but I want to note it here so we don't lose track of the feature request.

[garlick]

FLUX_TMPDIR is set to $FLUX_TMPDIR (or $TMPDIR, or /tmp) /flux-<session_id>-<rank>.
For an srun launch, <session_id> is set to the PMI "appnum" which slurm sets to job id.
For a direct launch, <session_id> is set to 0. What about changing that case to a uuid (provided by flux-start to all its children on the command line?)

[grondo]

Hm, commit 878361e didn't auto-close this issue. Perhaps the "Fixes Issue #..." needs to be on its own line? I also just learned that it is possible to close an issue from a pull request