logging should be unified

[garlick]

The log interfaces in libutil/log.[ch] should be deprecated.

flux_log() should be enhanced to:

• do something sensible when flux_t handle is NULL
• be configurable to log to stderr or syslog on any/all ranks in cases where the enclosing instance should be capturing the messages (e.g. LOG_CRIT or above?)
• provide errno decoding (including zeromq errno space) via %m or similar

oom() needs to be replaced with a configurable out of memory handler so ENOMEM can be dealt with appropriately for the situation.

flux_panic() and FASSERT() should be integrated with the overall logging strategy.

Possibly flux-start needs to capture stdio of the flux-broker processes in a local launch so that log messages to stderr from different ranks can be collated.

[garlick]

Some parts of this have been completed:

• libutil/log.[ch] has been stripped down and namespaced; what's left is still useful e.g. in utilities
• Flux logging (in the broker) is configurable to messages that exceed a severity threshold to stderr (see log-stderr-level attribute)
• flux-start uses libsubprocess to manage broker processes so it would be quite easy now to collate stdio (via subprocess zio) should that be desired

We still need to review flux_panic() and FASSERT(), and consider replacing oom() with a generic handler.

[garlick]

This issue is a bit broad and outdated. Closing, will reopen focused issues as needed.