Use uppercase prefix for kilo- and mega-

[JIghtuse]

Using 'mb/s' is confusing - user can thought it means 'millibit'. Since
we have a 'full notation', it is possible to use 'b' for bits and 'B'
for bytes. Let's use uppercase letter for prefix in this notation.

See also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=781488

[monsta]

Have you ever seen "millibits" anywhere? :)

[JIghtuse]

@monsta no, but still, using standardized units adds some readability. 😌

[monsta]

Ok this is a minor change anyway, let's merge it.

[piotrjurkiewicz]

With mb -> Mb you were right.

But kb should have stayed as it was. Small k should be used when kilo is 1000. Big K or Ki should be used when kilo is 1024. In this case, when bits == true, kilo is 1000 (see line 418). So small letter k should be used.

[JIghtuse]

@monsta should I send another PR to fix issue with 'k'?

[monsta]

I reverted kilo- prefix to lowercase in 908f931. Hope it's ok for you guys :)