fdisk: do not allow writing to a read-only device

Print a warning instead. This fixes the aforementioned "Bad file
descriptor" issue when writing to read-only completely.

Signed-off-by: Maciej Małecki <me@mmalecki.com>
libfdisk: remove `O_RDONLY` fallback mode when opening device
fdisk: retry opening device in read-only mode

Print a warning if opening in write mode fails.