ipcs: consolidate output header printing

ipcs: consolidate output header printing

Print a warning (instead of header) if --limits fails, like we did
it in past (2.20.1) and like we are still doing for --summary. Note
in past we were printing the same message like for --summary
"kernel not configured for ...", but actually this message is not
really correct.

This patch simply consolidates the current behavior. Probably we
should refactor it regarding warnings (stderr) and exit codes.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>