Add a Unix namespace for foo_t types.

This allows me to keep prototyping things on a random desktop machine,
even if that machine has its own ideas about foo_t types.

VirtualFileSystem/NullDevice.cpp

@@ -12,13 +12,13 @@ NullDevice::~NullDevice()
 {
 }
 
-ssize_t NullDevice::read(byte*, size_t)
+Unix::ssize_t NullDevice::read(byte*, Unix::size_t)
 {
     printf("read from null\n");
     return 0;
 }
 
-ssize_t NullDevice::write(const byte*, size_t bufferSize)
+Unix::ssize_t NullDevice::write(const byte*, Unix::size_t bufferSize)
 {
     return min(GoodBufferSize, bufferSize);
 }