Start working on memory-mapped files.

First of all, change sys$mmap to take a struct SC_mmap_params since our
sycsall calling convention can't handle more than 3 arguments.

This exposed a bug in Syscall::invoke() needing to use clobber lists.
It was a bit confusing to debug. :^)

VirtualFileSystem/UnixTypes.h

@@ -8,6 +8,17 @@ namespace Unix {
 #define SEEK_CUR 1
 #define SEEK_END 2
 
+#define MAP_SHARED 0x01
+#define MAP_PRIVATE 0x02
+#define MAP_FIXED 0x10
+#define MAP_ANONYMOUS 0x20
+#define MAP_ANON MAP_ANONYMOUS
+
+#define PROT_READ 0x1
+#define PROT_WRITE 0x2
+#define PROT_EXEC 0x4
+#define PROT_NONE 0x0
+
 typedef dword dev_t;
 typedef dword ino_t;
 typedef dword mode_t;