support multiple keys within single munged instance (aka multi-realm)

[dun]

Add support for a single munged instance to manage multiple keys, each for a different security realm. This would remove the need to run multiple munged processes concurrently, each bound to a unique socket and using a different key. The client would specify the realm via the MUNGE_OPT_REALM option.

Requested by Flux: because intergalactic flux instance might need to span clusters.