Fix incorrect summary duration during shutdown

[PartialVolume]

If the system is shutdown while nwipe is still
wiping, the duration calculation would be
incorrect. This patch fixes that problem, so for
instance if a UPS signals the system running nwipe to
shutdown, nwipe typically traps that signal and
exits in an orderly manner. The log summary
is generated showing the wipe was aborted and
the wipe duration shows a valid value i.e the
elapsed time since the start of the wipe.

Example of nwipe summary table after a shutdown is initiated mid wipe.

[2020/03/19 15:42:09] notice: Device /dev/sda excluded as per command line option -e
[2020/03/19 15:42:09] notice: Device /dev/sdb excluded as per command line option -e
[2020/03/19 15:42:09] notice: Found /dev/sdc, ATA WDC WD1600AAJS-2, 160GB, S/N=WD-WMAV2FS49552
[2020/03/19 15:42:09] notice: Device /dev/mapper/cryptswap1 excluded as per command line option -e
[2020/03/19 15:42:09] info: Automatically enumerated 1 devices.
[2020/03/19 15:42:09] notice: bios-version = E16F2IM7 V5.0E
[2020/03/19 15:42:09] notice: bios-release-date = 01/10/2012
[2020/03/19 15:42:09] notice: system-manufacturer = MEDION
[2020/03/19 15:42:09] notice: system-product-name = X681X
[2020/03/19 15:42:09] notice: system-version = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: system-serial-number = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: system-uuid = 03000200-0400-0500-0006-000700080009
[2020/03/19 15:42:09] notice: baseboard-manufacturer = MEDION
[2020/03/19 15:42:09] notice: baseboard-product-name = X681X
[2020/03/19 15:42:09] notice: baseboard-version = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: baseboard-serial-number = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: baseboard-asset-tag = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: chassis-manufacturer = To Be Filled By O.E.M.
[2020/03/19 15:42:09] notice: chassis-type = Notebook
[2020/03/19 15:42:09] notice: chassis-version = To be filled by O.E.M.
[2020/03/19 15:42:09] notice: chassis-serial-number = To Be Filled By O.E.M.
[2020/03/19 15:42:09] notice: chassis-asset-tag = To Be Filled By O.E.M.
[2020/03/19 15:42:09] notice: processor-family = Core i7
[2020/03/19 15:42:09] notice: processor-manufacturer = Intel
[2020/03/19 15:42:09] notice: processor-version = Intel(R) Core(TM) i7-2670QM CPU @ 2.20GHz
[2020/03/19 15:42:09] notice: processor-frequency = 800 MHz
[2020/03/19 15:42:09] notice: Opened entropy source '/dev/urandom'.
[2020/03/19 15:42:15] notice: /dev/sdc has serial number WD-WMAV2FS49552
[2020/03/19 15:42:15] notice: /dev/sdc, sect/blk/dev 512/4096/1128095744
[2020/03/19 15:42:15] notice: Invoking method 'DoD Short' on /dev/sdc
[2020/03/19 15:42:15] notice: Starting round 1 of 1 on /dev/sdc
[2020/03/19 15:42:15] notice: Starting pass 1/3, round 1/1, on /dev/sdc
[2020/03/19 15:42:47] error: Curses endwin() failed !
[2020/03/19 15:42:49] fatal: Nwipe exited with non fatal errors on device = /dev/sdc


********************************************************************************
! Device | Status | Thru-put | HH:MM:SS | Model/Serial Number
--------------------------------------------------------------------------------
!    sdc |UABORTED|   95MB/s | 00:00:34 | ATA WDC WD1600AAJ/WD-WMAV2FS49552
--------------------------------------------------------------------------------
[2020/03/19 15:42:49] Total Throughput   95MB/s, DoD Short, 1R+B+VL
********************************************************************************

Closes #21