select: use SA_RESTART with signals

Rely on the OS to automatically restart syscalls that were interrupted
by signals.  This makes code overall more robust because we don't have
to explicitly handle EINTR everywhere ourselves.

src/util/select.h

@@ -108,7 +108,7 @@ public:
     /* Register a handler, which will only be called when pselect()
        is interrupted by a (possibly queued) signal. */
     struct sigaction sa;
-    sa.sa_flags = 0;
+    sa.sa_flags = SA_RESTART;
     sa.sa_handler = &handle_signal;
     fatal_assert( 0 == sigfillset( &sa.sa_mask ) );
     fatal_assert( 0 == sigaction( signum, &sa, NULL ) );