attach: don't ignore sigint/sigkill if stdin is redirected

If attach is being done over passed-in fds, then we shouldn't
mess with the caller's signal table to ignore ctrl-c over the
fd.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>