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

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.

PR-URL: #10539
Fixes: #10520
Refs: #615
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: Stewart X Addison

src/node.cc

@@ -3957,6 +3957,7 @@ 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));
@@ -3970,6 +3971,7 @@ 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);