Add SA_RESTART flag to signal handler

With this flag, the kernel restarts automatically some system calls
that were interrupted by the signal. Without this flag, these system
calls would fail with errno = EINTR.

Even with this flag, the kernel cannot restart some system calls, and
developers should verify whether these system calls failed with EINTR
and restart them manually in that case. For the list of system calls
that cannot be restarted automatically, see signal(7).

Because of this, we cannot completely eliminate the side-effects of
liburcu on the application, but I think we should try our best and
set SA_RESTART.

From: Pierre-Marc Fournier <pierre-marc.fournier@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>