[PATCH 0/4] x86_64-gnu signal fixes

[Sergey Bugaev]

Hello,

now that we got enough of the system running on x86_64-gnu, we can
actually test the signal machinery. In fact we sort of *have to*,
because /bin/sh receives SIGCHLD when a command completes, and wants
to handle it.

Suprisingly, most of the logic actually does just work! -- but several
things were broken; and this patch series contains the fixes. With
these patches (+ the thread setup fixes that I'll send as a separate
series) I'm able to receive a signal, do all the trampoline and
intr-rpc magic, run the handler, and sigreturn back into user code --
apparently successfully.

Sergey