get_filename_component: Revise PROGRAM/PROGRAM_ARGS split semantics

The KWSys `SystemTools::SplitProgramFromArgs` implementation goes into
an infinite loop when the value is just " " (a space).  Since the
"program path with unquoted spaces plus command-line arguments"
operation it is trying to provide is poorly defined (string parsing
should not depend on filesystem content), just stop using it.

Instead consider the main two use cases the old approach tried to handle:

* The value is the name or absolute path of a program with no quoting
  or escaping, but also no command-line arguments.  In this case we
  can use the value as given with no parsing, and assume no arguments.

* The value is a command-line string containing the program name/path
  plus arguments.  In this case we now assume that the command line
  is properly quoted or escaped.

Fixes: #17262

Source/cmSystemTools.h

@@ -255,6 +255,11 @@ public:
   static void ParseUnixCommandLine(const char* command,
                                    std::vector<std::string>& args);
 
+  /** Split a command-line string into the parsed command and the unparsed
+      arguments.  Returns false on unfinished quoting or escaping.  */
+  static bool SplitProgramFromArgs(std::string const& command,
+                                   std::string& program, std::string& args);
+
   /**
    * Handle response file in an argument list and return a new argument list
    * **/