Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before.  Then revise the style:

* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`

Use the same format as CMake.  This is the Mozilla style with a few
tweaks:

* Do not align operator arguments.

* Do not always break after a function return type.

* Limit to 79 columns instead of 80 to fit edge cases
  better in 80-column terminals.

* Format for C++98 instead of C++11 because KWSys is written
  in the former language.

Change-Id: I755d3ca9cf2d883e85682fa86cab7127156da861