Added internal LFS_F_CANLOOKAHEAD flag

This is equivalent to the user-facing LFS_I_CANLOOKAHEAD flag, but
explicitly set in lfs_alloc/lfs_alloc_markfree, rather than being
implied.

Usually, I prefer implicit state, as this means less things that can
fall out-of-sync if there is a filesystem bug, but for
LFS_F_CANLOOKAHEAD explicit state might be warranted.

The main benefit is we can take advantage of the matching F/GC bit
patterns to simplify lfsr_fs_gc's progress checks.

This ends up saving a bit of code:

           code          stack
  before: 36048           2696
  after:  35988 (-0.2%)   2696 (+0.0%)

lfs.h

@@ -166,6 +166,8 @@ enum lfs_type {
 
 // internally used flags
 #define LFS_F_ORPHANS   0x01000000  // Filesystem may have untracked orphans
+#define LFS_F_CANLOOKAHEAD \
+                        0x02000000  // Lookahead buffer is not full
 #define LFS_F_UNCOMPACTED \
                         0x08000000  // Filesystem may have uncompacted metadata