Prefer explicit bitwise operations for modifying flags

The *_is* functions for testing for bit flags have proven useful, maybe
mostly due to C's bad bitwise operator precedence, but the
*_set*/*_clear* functions just add extra code without much benefit.

Relying on explicit bitwise operations also lets us use the |=/&= shortcut
operators, which are nice.

lfs.h

@@ -155,10 +155,8 @@ enum lfs_open_flags {
 #endif
 
     // internally used flags
-    // TODO do we need both unflushed and unsynced?
-    LFS_F_UNFLUSHED = 0x010000, // File's has data that needs to be written
-    LFS_F_UNSYNCED  = 0x020000, // File's metadata does not match storage
-    LFS_F_ERRORED   = 0x040000, // An error occurred during write
+    LFS_F_UNSYNCED  = 0x010000, // File's metadata does not match storage
+    LFS_F_ERRORED   = 0x020000, // An error occurred during write
 };
 
 // File seek flags