Kernel: Start implementing purgeable memory support

It's now possible to get purgeable memory by using mmap(MAP_PURGEABLE).
Purgeable memory has a "volatile" flag that can be set using madvise():

- madvise(..., MADV_SET_VOLATILE)
- madvise(..., MADV_SET_NONVOLATILE)

When in the "volatile" state, the kernel may take away the underlying
physical memory pages at any time, without notifying the owner.
This gives you a guilt discount when caching very large things. :^)

Setting a purgeable region to non-volatile will return whether or not
the memory has been taken away by the kernel while being volatile.
Basically, if madvise(..., MADV_SET_NONVOLATILE) returns 1, that means
the memory was purged while volatile, and whatever was in that piece
of memory needs to be reconstructed before use.

Kernel/UnixTypes.h

@@ -27,12 +27,16 @@
 #define MAP_ANONYMOUS 0x20
 #define MAP_ANON MAP_ANONYMOUS
 #define MAP_STACK 0x40
+#define MAP_PURGEABLE 0x80
 
 #define PROT_READ 0x1
 #define PROT_WRITE 0x2
 #define PROT_EXEC 0x4
 #define PROT_NONE 0x0
 
+#define MADV_SET_VOLATILE 0x100
+#define MADV_SET_NONVOLATILE 0x200
+
 #define F_DUPFD 0
 #define F_GETFD 1
 #define F_SETFD 2