Provide a way to inject network credentials into PM startup

DRC credentials cannot be obtained more than once per slurm allocation. We should provide a way to obtain the network credential part of the PMINFO data separately, and override the allocation phase of PM startup. This is useful for @biddiscombe1 and @dabin1.

We will introduce a MSTRO_NETWORK_CREDENTIAL environment variable that contains a string of the form "kind:value" (with drc:<numer> for now as the only supported kind). That can be taken into account by the DRC init code. To obtain the value a new accessor will be implemented, mstro_pm_getnetcred(char **result_p) in pool_manager.h that returns this kind of data.

This design seems preferrable to re-using the pminfo block (which already contains the credential), as it will allow fully-external injection of credentials, rather than only allowing re-use of the same allocation for multiple pool-manager startups. It does not preclude adding the pminfo-reuse semantics later on.