HostDiskUsageService

[KernelDeimos]

Right now we're using the diskusage package, added in this PR, to measure the host OS disk capacity (to display in Puter's storage usage meter). This package uses node-gyp and depends on Python, so it limits portability. We could write a our own in node from scratch with implementations for POSIX, OSX, and Windows to start off with.

Checking disk capacity should happen in two phases:

• What mountpoint/drive is the current process working directory under?
• What is the capacity of that mountpoint/drive?

Someone who only uses one of the three operating systems I listed could start it off and leave blanks for the next person to fill; let's just make sure the "unsupported operating system" case is something pragmatic, like "log a notice that it's not supported, and set apparent disk capacity to an arbitrary large number"