Renamed scratch files -> orphan files

I was originally avoiding naming these orphans, as they're _technically_
not orphans. They do exist in the mtree. But the name orphan just
describes this types purpose too well.

This does lead to some confusing terms, such as the fact that orphan
files can be non-orphaned if there are any in-device references. But I
think this makes sense?

- LFSR_TAG_SCRATCH -> LFSR_TAG_ORPHAN
- LFSR_F_UNCREAT -> LFSR_F_ORPHAN
- test_fscratch.toml -> test_forphan.toml

lfs.h

@@ -139,7 +139,7 @@ enum lfs_open_flags {
     // internally used flags
     LFS_F_UNFLUSH   = 0x1000,   // File's data does not match storage
     LFS_F_UNSYNC    = 0x2000,   // File's metadata does not match storage
-    LFS_F_UNCREAT   = 0x4000,   // File does not exist yet
+    LFS_F_ORPHAN    = 0x4000,   // File does not exist yet
     LFS_F_ZOMBIE    = 0x8000,   // File has been removed
 };