build: don't squash signal handlers with --shared

Fixes: nodejs#10520
Ref: nodejs@dd47a8c

An application using node built as a shared library may legitimately
implement its own signal handling routines. Current behaviour is
to squash all signal handlers on node startup. This change will
stop that behaviour when node is built as a shared library.

src/node.cc

@@ -2192,7 +2192,7 @@ static void WaitForInspectorDisconnect(Environment* env) {
   if (env->inspector_agent()->IsConnected()) {
     // Restore signal dispositions, the app is done and is no longer
     // capable of handling signals.
-#ifdef __POSIX__
+#if defined(__POSIX__) && !defined(NODE_SHARED_MODE)
     struct sigaction act;
     memset(&act, 0, sizeof(act));
     for (unsigned nr = 1; nr < kMaxSignal; nr += 1) {
@@ -4093,6 +4093,7 @@ inline void PlatformInit() {
 
   CHECK_EQ(err, 0);
 
+#ifndef NODE_SHARED_MODE
   // Restore signal dispositions, the parent process may have changed them.
   struct sigaction act;
   memset(&act, 0, sizeof(act));
@@ -4106,6 +4107,7 @@ inline void PlatformInit() {
     act.sa_handler = (nr == SIGPIPE) ? SIG_IGN : SIG_DFL;
     CHECK_EQ(0, sigaction(nr, &act, nullptr));
   }
+#endif  // !NODE_SHARED_MODE
 
   RegisterSignalHandler(SIGINT, SignalExit, true);
   RegisterSignalHandler(SIGTERM, SignalExit, true);