From: Mark A. Grondona <mgrondona@llnl.gov>

Certain SPANK items that are available in local or allocator context
may not be available until after cmdline options have been processed,
or after a job has been allocated. Return ESPANK_NOT_AVAIL to indicate
to plugins that this is the case.

This patch fixes a potential segfault if SPANK plugins call spank_get_item
for these items from early callbacks such as spank_init.