Bug#870364: say "... is already mounted rw elsewhere. Cannot mount ro."

[積丹尼 Dan Jacobson]

Package: mount
Version: 2.29.2-2
Severity: wishlist

# mount -o ro /dev/sda7 /mnt/usb/extra/
mount: /dev/sda7 is already mounted or /mnt/usb/extra busy
       /dev/sda7 is already mounted on /
# mount  /dev/sda7 /mnt/usb/extra/
#

So it should say instead:
mount: /dev/sda7 is already mounted rw elsewhere. Cannot mount ro.