fdisk: fix -l [<device>]

 * don't use err(), but warn(), so don't stop after the first error

 * display all errors when "-l <device> ..." specified

 * display only EACCES errors when list whole disks from
   /proc/partitions ("fdisk -l" )

Reported-by: Boian Bonev <bbonev@ipacct.com>
Addresses: https://github.com/karelzak/util-linux/pull/106
Signed-off-by: Karel Zak <kzak@redhat.com>