Fixes logging suppression caused by early logging.

[rountree]

Logging initialization assumes a missing SPINDLE_DEBUG environment variable means SPINDLE_DEBUG=0. Messages that are logged prior to that environment variable being set are lost and inadvertently suppress any further logging messages from that subsystem.

This patch sets SPINDLE_DEBUG=0 in the shell environment if that variable is missing from the flux environment. It also modifies the logging initialization process so that any messages logged before SPINDLE_DEBUG is set are sent through, but any subsequent message will trigger re-initialization until SPINDLE_DEBUG exists.

Fixes #55

[mplegendre]

We need to leave debugging off if SPINDLE_DEBUG is unset. There's significant overhead/poor-scaling if debugging is enable. And file pollution, as it drops files in your PWD.
The typical way users will run will be without SPINDLE_DEBUG set. There's also several run modes that don't go through the flux plugin, so we can't just rely on setting/unsetting it there.

If you're going to insert a compile-time instrumentation that requires debug tracing, then add a compile-time flag that turns debugging on by default. I can help add that flag, if that would be easier.

[rountree]

Point taken wrt scaling and file pollution. I can imagine a solution where messages (with their priority) are buffered until initialization occurs, then parsed to see if they're logged or discarded. It's not wholly unlike trying to make printf() work in _start() before your file descriptors are set up, and, perhaps like that, it's not a problem worth solving (or not worth solving this week).

[mplegendre]

But then you've got to decide how long to buffer. And you're still paying the internal overhead of logging.

I've got a upcoming commit that changes Spindle's config management. It's already got some other changes to autoconf configure. I'll throw in a configure --with-default-debugging-level=X with that commit, and we can set that when adding the -finstrument.

I'm going to close this. We can re-open if the discussion circles back to this kind of approach.