Apple2: Provide a way to get directory file count

The information is available in the directory key block.
Providing it to the user as soon as opendir() is done
can save them costly code.

libsrc/apple2/opendir.s

@@ -128,24 +128,23 @@
         ; Read succeeded, populate dir struct
         jsr       popptr1     ; Restore our dir pointer
 
-        ldy       #$24 + DIR::BYTES
-        lda       (ptr1),y    ; ENTRIES_PER_BLOCK
-        pha                   ; Back it up
-
+        ; Get file_count to entry_length from block
+        ldy       #$26 + DIR::BYTES
+:       lda       (ptr1),y
+        pha
         dey
-        lda       (ptr1),y    ; ENTRY_LENGTH
+        cpy       #$23 + DIR::BYTES - 1
+        bne       :-
 
+        ; Set entry_length to file_count in struct
         ldy       #DIR::ENTRY_LENGTH
+:       pla
         sta       (ptr1),y
-
-        pla
-        .assert   DIR::ENTRIES_PER_BLOCK = DIR::ENTRY_LENGTH + 1, error
         iny
-        sta       (ptr1),y
+        cpy       #DIR::CURRENT_ENTRY
+        bne       :-
 
         ; Skip directory header entry
-        .assert   DIR::CURRENT_ENTRY = DIR::ENTRIES_PER_BLOCK + 1, error
-        iny
         lda       #$01
         sta       (ptr1),y