most of set_current_blocked() callers want SIGKILL/SIGSTOP removed from set

Only 3 out of 63 do not.  Renamed the current variant to __set_current_blocked(),
added set_current_blocked() that will exclude unblockable signals, switched
open-coded instances to it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/powerpc/kernel/signal.c

@@ -57,7 +57,6 @@ void __user * get_sigframe(struct k_sigaction *ka, struct pt_regs *regs,
  */
 void restore_sigmask(sigset_t *set)
 {
-	sigdelsetmask(set, ~_BLOCKABLE);
 	set_current_blocked(set);
 }