Add close-on-exec flag for file descriptors.

I was surprised to find that dup()'ed fds don't share the close-on-exec flag.
That means it has to be stored separately from the FileDescriptor object.

VirtualFileSystem/VirtualFileSystem.h

@@ -14,8 +14,15 @@
 #define O_RDONLY 0
 #define O_WRONLY 1
 #define O_RDWR 2
+#define O_CREAT 0100
+#define O_EXCL 0200
+#define O_NOCTTY 0400
+#define O_TRUNC 01000
+#define O_APPEND 02000
+#define O_NONBLOCK 04000
 #define O_DIRECTORY 00200000
 #define O_NOFOLLOW 00400000
+#define O_CLOEXEC 02000000
 #define O_NOFOLLOW_NOERROR 0x4000000
 
 class CharacterDevice;