Fix issue 272 - error when echo 0 into dirty_background_bytes and dirty_bytes

[przemyslaw-szustak]

When
echo 0 > /proc/sys/vm/dirty_background_bytes
or
echo 0 > /proc/sys/vm/dirty_bytes
failed and return 1 (file was probably locked because it existed and was writable) then woeusb crashed with message 'program is prematurely aborted' but everything works.

[slacka]

Looks good. Thanks for the fix!

[SpineEyE]

I'm not sure what this workaround is about, but those commands will actually never run successfully:

# echo 0 > /proc/sys/vm/dirty_bytes
bash: echo: write error: Invalid argument
# echo 0 > /proc/sys/vm/dirty_background_bytes
bash: echo: write error: Invalid argument

See https://www.kernel.org/doc/Documentation/sysctl/vm.txt

[slacka]

@SpineEyE
Indeed, these values are read-only on current kernels. However this was originally added to work around a kernel bug. See 0f1d3a2