Allow creation of persistent shadow copies about shadowspawn

[candera]

I'm not inclined to do this one at this point. It would require specifying what are called persistent shadow copies. Persistent shadow copies are only possible against NTFS, and don't work on Windows XP, so that would break our cross-OS support story, or at least require us to document the different behaviors.

If I were going to do this, I would probably take the approach of splitting ShadowSpawn into a library for working with shadow copies and two executables: one that would be what shadowspawn is now, and one to do what you want. I tend to prefer tools that just do one thing.

There are two ways to achieve this with existing tools:

1. Launch cmd.exe instead of robocopy, and use the /c switch to specify a batch file. The batch file can have as many commands as you want in it.
2. Use vshadow.exe - I'm pretty sure it does this already.