spank: fix handling of remote spank_init_post_opt

There was likely a typo/thinko/patcho in the handling of the
return code from _do_call_stack(SPANK_INIT_POST_OPT) in _spank_init
in "remote" context. This error caused spank_init() to always
succeed, since the test less than zero would always return 0 or 1.