Replace the "node" shell command in shell.js with process.execPath

On platforms like Ubuntu/Debian where the node command has been taken by a throwback command (node(8) - Node front end for AX.25, NET/ROM, Rose and TCP), the shell js exec goes into an infinite loop because the node command exits with status 1.

I ran into this error while figuring out how to build pdf.js 

https://github.com/mozilla/pdf.js/pull/2608

I use process.execPath to invoke the node command again, so that it picks up and runs the sub command with "/usr/bin/nodejs" instead of "node".

This had to be quoted again for the shell, to accommodate win32 versions as well which need to run "C:\Program Files\" which has a space in it.