more debugging output help if margo-info segfaults

[carns]

Motivating use case reported by @soumagne . The margo-info utility redirects stderr before iterating through known address types to probe for system support. If one of these calls segfaults (e.g., in a communication library beyond margo's control) then margo-info exits without any indication on the command line, and even the segfault message may not be displayed either because apparently it too is consumed by stderr redirection.

We should find a way to display some incremental progress or else simply identify the intermediate redirection files in /tmp early on so that the user has a way to diagnose what went wrong in this scenario.