Verify plugin versions at load time

Verify that all plugin version numbers are identical to the component
attempting to load them. Without this verification, the plugin can reference
Slurm functions in the caller which differ (e.g. the underlying function's
arguments could have changed between Slurm versions).
NOTE: All plugins (except SPANK) must be built against the identical
version of Slurm in order to be used by any Slurm command or daemon. This
should eliminate some very difficult to diagnose problems due to use of old
plugins.