This fixes all known signal semantics problems.

sigwait() is really evil - i had to re-introduce ->real_blocked. When a
signal has no handler defined then the actual action taken by the kernel
depends on whether the sigwait()-ing thread was blocking the signal
originally or not. If the signal was blocked => specific delivery to the
thread, if the signal was not blocked => kill-all.

fortunately this meant that PF_SIGWAIT could be killed - the real_blocked
field contains all the necessery information to do the right decision at
signal-sending time.

i've also cleaned up and made the shared-pending code more robust: now
there's a single central dequeue_signal() function that handles all the
details. Plus upon unqueueing a shared-pending signal we now re-queue the
signal to the current thread, which this time around is not going to end
up in the shared-pending queue. This change handles the following case
correctly: a signal was blocked in every signal, then one thread unblocks
it and gets the signal delivered - but there's no handler for the signal
=> the correct action is to do a kill-all.

i removed the unused shared_unblocked field as well, reported by Oleg
Nesterov.

now we pass both signal-tst1 and signal-tst2, so i'm confident that we got
most of the details right.