detect PRIVATE/M4ROOT and return the requested store count (closes #6)

A card was detected only with M4ROOT at its root, and asking for two or
more stores returned one too few. A mount point now counts when its root
holds a DCIM or PRIVATE/M4ROOT directory, and the count is exact.

The check and the counting are split from the partition listing so tests
can hand them temporary directories.

Model: opus-5-5

README.md

@@ -116,8 +116,8 @@ The main types are:
 - `Image` and `Video` embed `Object`. The file extension decides which one a
   file becomes.
 
-A mounted filesystem is a DCF store when its root holds a `DCIM` or `M4ROOT`
-directory. `dcf` never mounts a filesystem.
+A mounted filesystem is a DCF store when its root holds a `DCIM` or
+`PRIVATE/M4ROOT` directory. `dcf` never mounts a filesystem.
 
 ## TODO
 
@@ -126,13 +126,9 @@ directory. `dcf` never mounts a filesystem.
   tar of a memory card that uses them, and I'll see what I can do.
 - Parse the DCF directory and file names, rather than listing every media file
   on the filesystem.
-- Look for `PRIVATE/M4ROOT` rather than `M4ROOT` when detecting a DCF store.
-- Return exactly the number of stores asked for. `GetDCFStores` returns one too
-  few when asked for two or more.
 - Read stores through `fs.FS`, with a `context.Context` to cancel a walk.
 - Return a plain slice from `GetDCFStores`, not a pointer to one.
-- Test the walk of a store and the detection of mount points. They only run
-  against real mounted media today.
+- Test the walk of a store. It only runs against real mounted media today.
 - Copy files off a card, and verify each copy against its SHA256 hash.
 
 ## License