child_process: set shell to false in fork()

This commit ensures that spawn()'s shell option is unconditionally
set to false when fork() is called.

Refs: nodejs#15299
Fixes: nodejs#13983
PR-URL: nodejs#15352
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/child_process.md

@@ -356,6 +356,9 @@ output on this fd is expected to be line delimited JSON objects.
 *Note*: Unlike the fork(2) POSIX system call, `child_process.fork()` does
 not clone the current process.
 
+*Note*: The `shell` option available in [`child_process.spawn()`][] is not
+supported by `child_process.fork()` and will be ignored if set.
+
 ### child_process.spawn(command[, args][, options])
 <!-- YAML
 added: v0.1.90

lib/child_process.js

@@ -93,6 +93,7 @@ exports.fork = function(modulePath /*, args, options*/) {
   }
 
   options.execPath = options.execPath || process.execPath;
+  options.shell = false;
 
   return spawn(options.execPath, args, options);
 };