process: save original argv[0]

For historical and other reasons, node overwrites `argv[0]` on startup.
See
 - 2c6f79c,
 - #7434
 - #7449
 - #7696

For cases where it may be useful, save the original value of `argv[0]`
in `process.argv0`

PR-URL: #7696
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/process.md

@@ -457,9 +457,10 @@ added: v0.1.27
 
 The `process.argv` property returns an array containing the command line
 arguments passed when the Node.js process was launched. The first element will
-be [`process.execPath`]. The second element will be the path to the
-JavaScript file being executed. The remaining elements will be any additional
-command line arguments.
+be [`process.execPath`]. See `process.argv0` if access to the original value of
+`argv[0]` is needed.  The second element will be the path to the JavaScript
+file being executed. The remaining elements will be any additional command line
+arguments.
 
 For example, assuming the following script for `process-args.js`:
 
@@ -486,6 +487,22 @@ Would generate the output:
 4: four
 ```
 
+## process.argv0
+<!-- YAML
+added: REPLACEME
+-->
+
+The `process.argv0` property stores a read-only copy of the original value of
+`argv[0]` passed when Node.js starts.
+
+```js
+$ bash -c 'exec -a customArgv0 ./node'
+> process.argv[0]
+'/Volumes/code/external/node/out/Release/node'
+> process.argv0
+'customArgv0'
+```
+
 ## process.chdir(directory)
 <!-- YAML
 added: v0.1.17

lib/internal/bootstrap_node.js

@@ -50,6 +50,11 @@
 
     _process.setupRawDebug();
 
+    Object.defineProperty(process, 'argv0', {
+      enumerable: true,
+      configurable: false,
+      value: process.argv[0]
+    });
     process.argv[0] = process.execPath;
 
     // There are various modes that Node can run in. The most common two