Replace KB unit with KiB (#4293)

kB (*kilo* byte) unit means 1000 bytes, whereas KiB ("kibibyte")
means 1024 bytes. KB was misused: replace kB or KB with KiB when
appropriate.

Same change for MB and GB which become MiB and GiB.

Change the output of Tools/iobench/iobench.py.

Round also the size of the documentation from 5.5 MB to 5 MiB.
diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index 4999fa7..dab17c3 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -96,7 +96,7 @@
 
     largefile - It is okay to run some test that may create huge
                 files.  These tests can take a long time and may
-                consume >2GB of disk space temporarily.
+                consume >2 GiB of disk space temporarily.
 
     network -   It is okay to run tests that use external network
                 resource, e.g. testing SSL support for sockets.