signals: protect cinit from unblocked SIG_DFL signals

Drop early any SIG_DFL or SIG_IGN signals to container-init from within
the same container.  But queue SIGSTOP and SIGKILL to the container-init
if they are from an ancestor container.

Blocked, fatal signals (i.e when SIG_DFL is to terminate) from within the
container can still terminate the container-init.  That will be addressed
in the next patch.

Note:	To be bisect-safe, SIGNAL_UNKILLABLE will be set for container-inits
   	in a follow-on patch. Until then, this patch is just a preparatory
	step.

Signed-off-by: Sukadev Bhattiprolu <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Roland McGrath <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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