Add epic_fs_is_attached(), os.fs_is_attached(), and make the menu automatically return from USB mode

Add a new API call to check whether the filesystem is attached to card10 or to a connected USB host (in which case it is unavailable to card10). Make the menu check this and automatically exit "USB storage mode" when a host ejects the device.

Documentation/pycardium/os.rst

@@ -115,3 +115,17 @@ Card10-Specific
 .. py:data:: USB_FLASH
 
    Mass-Storage device active.
+
+.. py:function:: fs_is_attached()
+
+   Check whether the filesystem is currently attached to card10 (or whether a connected
+   USB host is currently holding control over it and possibly writing to it).
+
+   :returns:
+
+      - ``True`` if the filesystem is attached to card10 and an app can read and
+        write files.
+      - ``False`` if the filesystem is not available to card10 because a USB
+        host is currently controlling it.
+
+   .. versionadded: 1.18