get the "-unique" flag and [pd~] object working in Purr Data

This patch makes it possible to have multiple running instances of Purr Data, and to leverage that interface from Pd-l2ork to get [pd~] working on all platforms

Unfortunately there were so many overlaps between properly implementing "-unique" and getting [pd~] working cross-platform that it was too complicated to tease out the commits into two separate merges. Hence this monster patch, squashed down to make it easier to manage in the tree.

Some difficulties required revisions to [pd~] in Pd Vanilla's code

• use quoted path args for Windows when starting the [pd~] subprocess
• replace sys_havegui() from Pd Vanilla's recent "threadening" with sys_nogui

The "-unique" flag requires the OSX app bundle to be able to start by calling the binary in Pd-l2ork.app/Contents/Resources/app.nw/bin/pd-l2ork. This should also make it possible to run Purr Data on OSX both by using an app bundle (where the GUI starts the Pd process) and by installing and running from the command line (where the Pd process starts the GUI)

The "--user-data-dir" flag sets the directory for various nw.js state used by a running instance. To prevent races nw.js requires that each running instance have its own directory that is not shared with any other instances.

Setting the value for this flag has two important potential side-effects:

1. If you have a running instance and try to start an additional instance that has the same data directory, nw.js will just send an "open" event the first running instance and exit the 2nd instance gracefully. This is the default nw.js behavior and the default Pd-l2ork behavior as well.
2. If you have a running instance and try to start an additional instance with a different data directory than the first, you'll get a fully functional 2nd instance of nw.js running. This is the behavior we desire when the user runs pd-l2ork with the "-unique" flag.

In each case we set the data dir explicitly in order to get consistent positional command line arguments in the GUI.

In normal mode Purr Data will use the default data dir as specified in the nw.js docs for the particular platorm. Since the same default dir is used each time Purr Data is run, there will be only one data dir that gets created.

When run with the "-unique" flag, Purr Data will append "-$portno" to the default data directory, where $portno is the port Pd chose to communicate with the GUI. That ensures that each separate instance of Purr Data can get a unique data dir.

A side effect is that users may end up generating multiple data dirs that don't get deleted on exit. But Pd starts searching for available ports in the same range each time, and unless the user tries to run hundreds of instances at once this directory spam shouldn't be too big of a problem.

Changes to [stdout] still need to be ported from Pd Vanilla.

Once this completes successfully on all platforms it should be ready to merge.