Remove redundant null checks on sched_params.

xstrcasestr() knows how to handle NULL on any argument, so these
are all redundant checks. Cleanup related handling code while here,
and convert a few missed strstr/strcasestr locations as well.

In job_submit_throttle plugin, use sched_params as the name for
consistency with the rest of the code base.

Update the newly added bf_ignore_newly_avail_nodes option to use
xstrcasestr() like the rest of these options.

Bug 5976.