execute_process: Restore no-decoding default behavior

Since commit v3.8.0-rc1~232^2 (execute_process: Add ENCODING option for
Windows child process output, 2016-11-23) we decode child process output
using the console's active codepage by default.  This differs from
previous versions of CMake and is therefore incompatible.  Changing this
default will require a policy, so for now revert the default behavior
back to performing no decoding.

Reported-by: Nils Gladitz <nilsgladitz@gmail.com>

Help/command/execute_process.rst

@@ -75,10 +75,10 @@ Options:
  ``NONE``
    Perform no decoding.  This assumes that the process output is encoded
    in the same way as CMake's internal encoding (UTF-8).
+   This is the default.
  ``AUTO``
    Use the current active console's codepage or if that isn't
    available then use ANSI.
-   This is the default.
  ``ANSI``
    Use the ANSI codepage.
  ``OEM``

Source/cmExecuteProcessCommand.cxx

@@ -47,7 +47,7 @@ bool cmExecuteProcessCommand::InitialPass(std::vector<std::string> const& args,
   std::string error_variable;
   std::string result_variable;
   std::string working_directory;
-  cmProcessOutput::Encoding encoding = cmProcessOutput::Auto;
+  cmProcessOutput::Encoding encoding = cmProcessOutput::None;
   for (size_t i = 0; i < args.size(); ++i) {
     if (args[i] == "COMMAND") {
       doing_command = true;