This has been a FIXME for a long time. We now apply the provided
read/write permissions to the constructed FileDescription when opening
a File object via File::open().
After reading a bunch of POSIX specs, I've learned that a file descriptor
is the number that refers to a file description, not the description itself.
So this patch renames FileDescriptor to FileDescription, and Process now has
FileDescription* file_description(int fd).
