Select/cray - Modify build to enable direct use of libslurm library.

These changes are required so that select/cray can load select/linear,
  which is a bit more complex than the other select plugin structures.
Export plugin_context_create and plugin_context_destroy symbols from
  libslurm.so.
Correct typo in exported hostlist_sort symbol name
Define some functions in select/cray to avoid undefined symbols if
  the plugin is loaded via libslurm rather than from a slurm command
  (which has all of the required symbols)