Allow "source" to load python scripts.

* exceptions.h (enum errors): Add UNSUPPORTED_ERROR.
        * python/python.c (source_python_script): New function.
        * python/python.h (source_python_script): Add declaration.
        * cli/cli-cmds.c: #include exceptions.h and python/python.h.
        (script_ext_off, script_ext_soft, script_ext_strict)
        (script_ext_enums, script_ext_mode): New static constants.
        (show_script_ext_mode, find_and_open_script): New functions.
        (source_script): Enhance to handle Python scripts.
        (init_cli_cmds): Add set/show script-extension commands.

gdb/exceptions.h

@@ -75,6 +75,9 @@ enum errors {
   /* Error accessing memory.  */
   MEMORY_ERROR,
 
+  /* Feature is not supported in this copy of GDB.  */
+  UNSUPPORTED_ERROR,
+
   /* Add more errors here.  */
   NR_ERRORS
 };