Add option to 'growpart' to specify percentage of device that should be
left unallocated when growing partition. This is intended for consumer
SSDs and SD cards where the performance and/or lifetime of these devices
can be improved if some disk space (in addition to any the device "hides"
from users) is left unallocated.

Overprovisioning code caters for several distinct scenarios:

(1) MSDOS/MBR partitioned disk where the disk is >2TB and so MBR
    partitions cannot extend beyond 2TB - if disk is larger than
    (2TB + overprovisioning requirement) then nothing needs to be
    done.

(2) MSDOS/MBR partitioned disk where the disk is >2TB and so MBR
    partitions cannot extend beyond 2TB - if disk is not larger
    than (2TB + overprovisioning requirement) then *some*
    overprovisioning space still needs to be reserved.

(3) MSDOS/MBR partitioned disk <=2TB where overprovisioning space
    needs to be reserved.

(4) GPT partitioned disk where overprovisioning space needs to be
    reserved.

Also added a testcase script, test-growpart-overprovision.

Correct some of the overprovisioning logic.

Change overprovisioning testcase to not require root.

Also correct some off-by-one errors in the existing growpart code and
correct some existing testcases affected by this.