srun: Prevent shepard proc from calling spank_fini()

The spank_fini() function is registered with atexit() to be called
after termination of the srun main() function. The registered
functions are inherited by the forked shepard process and thus
spank_fini() is called twice.
This commit fixes this problem by introducing a wrapper function
_call_spank_fini() that is a no-op in the context of the shepard
process.