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/dir.h

@@ -45,6 +45,7 @@ struct DIR {
     int           fd;
     unsigned char entry_length;
     unsigned char entries_per_block;
+    unsigned int  file_count;
     unsigned char current_entry;
     union {
         unsigned char bytes[512];