Add logic to display disks with no partitions

Normally we don't want to see a 'disk', but if it has no partitions (eg,
internal storage on some portable media devices) then it should be visible.
3.1.0
In the function "list_devices", replace grep with regex when looking for
partitions on a device.
Only the presence of /dev/sda1 has to be checked, as /dev/sda2 implies the presence of /dev/sda1.
This could be improved further by only checking if the next array element is /dev/sda1, as the array is already sorted.