Support basic mmap'ing of a file!

All right, we can now mmap() a file and it gets magically paged in from fs
in response to an NP page fault. This is really cool :^)

I need to refactor this to support sharing of read-only file-backed pages,
but it's cool to just have something working.

LibC/mman.h

@@ -14,6 +14,8 @@
 #define PROT_EXEC 0x4
 #define PROT_NONE 0x0
 
+#define MAP_FAILED ((void*)-1)
+
 __BEGIN_DECLS
 
 void* mmap(void* addr, size_t, int prot, int flags, int fd, off_t);