Adopted common O_RDONLY/WRONLY/RDWR bit patterns

This should, in theory, be a transparent change for users
(https://xkcd.com/1172).

The motivation for this change:

1. Basically everyone uses O_RDONLY=0, O_WRONLY=1, O_RDWR=2, so
   deviating from this ad-hoc standard risks surprising POSIX-familiar
   users, though may confused POSIX-unfamiliar users.

   But for the latter, we really shouldn't allow them to fall into the
   trap that O_RDONLY | O_WRONLY == O_RDWR, because this will not work
   on basically any other POSIX-like system.

2. I realized one benefit of the POSIX encoding is that it reserves the
   value 3. Maybe this could be useful in the future?

   Being able to create a file that neither readable nor writable isn't
   all that useful...

Also, if you really think about the literal meaning of O_RDONLY |
O_WRONLY, these are negations. So O_RDONLY | O_WRONLY means you can only
write and only read? That sounds like an oxymoron.

Of course no one should be relying on these exact values, but these are
embedded systems! Someone somewhere is going to hack something together
that expects these to be their historically expected value. And we
shouldn't make things any harder for them unless there's a good reason.

lfs.h

@@ -123,10 +123,10 @@ enum lfs_type {
 // File open flags
 enum lfs_open_flags {
     // open flags
-    LFS_O_RDONLY = 1,         // Open a file as read only
+    LFS_O_RDONLY = 0,         // Open a file as read only
 #ifndef LFS_READONLY
-    LFS_O_WRONLY = 2,         // Open a file as write only
-    LFS_O_RDWR   = 3,         // Open a file as read and write
+    LFS_O_WRONLY = 1,         // Open a file as write only
+    LFS_O_RDWR   = 2,         // Open a file as read and write
     LFS_O_CREAT  = 0x0004,    // Create a file if it does not exist
     LFS_O_EXCL   = 0x0008,    // Fail if a file already exists
     LFS_O_TRUNC  = 0x0010,    // Truncate the existing file to zero size