reporter: simplify --configreport handling for -S|--select and fix an issue reported...

reporter: simplify --configreport handling for -S|--select and fix an issue reported by coverity

Simplify code around _do_get_report_selection - remove "expected_idxs[]"
argument which is superfluous and add "allow_single" switch instead to
allow for recognition of "--configreport <report_name> -S" as well as
single "-S" if needed.

Null pointer dereferences  (FORWARD_NULL) /safe/guest2/covscan/LVM2.2.02.158/tools/reporter.c: 961 in _do_report_get_selection()
Null pointer dereferences  (FORWARD_NULL) Dereferencing null pointer "single_args".