terminal: always use SA_RESTART with sigaction()

One problem is that for example stdio functions won't restart syscalls
manually, and instead treat EINTR as an error. So passing SA_RESTART is
the only sane thing to do, unless you have special requirements, which
we don't.

osdep/terminal-unix.c

@@ -518,7 +518,7 @@ static int setsigaction(int signo, void (*handler) (int),
     else
         sigemptyset(&sa.sa_mask);
 
-    sa.sa_flags = flags;
+    sa.sa_flags = flags | SA_RESTART;
     return sigaction(signo, &sa, NULL);
 }