Help: Document ctest_start initial checkout command variables

Describe the behavior of the ctest_start() command when the
CTEST_CHECKOUT_COMMAND or CTEST_CVS_CHECKOUT variable is set.
Help: Document ctest dashboard client usage

Extend the ctest(1) manual with a section describing configuration
and usage of CTest as a CDash software quality dashboard client.
Help: Add missing space in policy CMP0020 description
Help: Format cmake_policy command documentation

Add reStructuredText markup manually to improve formatting of the
documentation.  Organize the documentation into subsections.
Help: Reference cmake_minimum_required from cmake_policy docs

Note in a few places of the cmake_policy() command documentation
that the cmake_minimum_required() command can set policies too.

Inspired-by: Peter Kümmel <syntheticpp@gmx.net>
Features: Enable compiler extensions by default.

Compilers enable their extensions by default, and disabling them
implicitly can lead to results which are surprising or non-obvious
to debug.

 http://public.kitware.com/pipermail/cmake-developers/2014-May/010575.html
 http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/10214
 https://www.mail-archive.com/cmake-developers@cmake.org/msg10116.html
 (Compiler feature extensions by default, 29 May 2014)
Help: Add code example for INSTALL_INTERFACE to cmake-buildsystem.
Merge topic 'backport-doc-include-dirs-example'

6fbd9a8f Help: Add code example for INSTALL_INTERFACE to cmake-buildsystem.
Merge topic 'policy-doc-updates'

b1f4e620 Help: Reference cmake_minimum_required from cmake_policy docs
69a23cb2 Help: Format cmake_policy command documentation
0c4835a3 Help: Add missing space in policy CMP0020 description
Merge topic 'doc-ctest-settings'

2d57a390 Help: Document ctest dashboard client usage
28f73dff Help: Document ctest_start initial checkout command variables
Merge topic 'feature-extensions-by-default'

60a981ea Features: Enable compiler extensions by default.