Hi,
I've written a tiny patch that should fix this issue with su:
https://bugzilla.redhat.com/show_bug.cgi?id=747592.
None of the shells' output propagates back
to the shell where su was called (except TCSH, which reports segfault
to stdout; I've tested this with sh,ksh,zsh,bash).
So, with this patch, su should print a warning if
WIFSIGNALED(status) && WCOREDUMP(status) is true.

Cheers,
  Ondrej