fdisk: backport disk check from util-linux

fdisk: backport disk check from util-linux

With the digit check devices like mmcblk0 were skipped,
but now with 0 allowed we're seeing a ton of loop devices listed
(loop0, loop10, loop20...) as well as ramzswap0,
all which should not be shown in fdisk -l.

function                                             old     new   delta
list_devs_in_proc_partititons                        157     238     +81

Signed-off-by: Lauri Kasanen <curaga@operamail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>