Add a new "python-interactive" command that starts a standard

	Add a new "python-interactive" command that starts a standard
	Python interactive prompt with "pi" as alias, and add "py" as
	an alias to "python".
	* NEWS: Mention the new commands.
	* doc/gdb.texinfo (Python Commands): Document the new
	commands.
	* python/python.c (eval_python_command): New function.
	(python_interactive_command): For "python-interactive" with
	arguments, call eval_python_command.  For "python-interactive"
	without arguments, call PyRun_InteractiveLoop.
	(_initialize_python): Add "python-interactive" command with
	"pi" as alias, and add "py" as an alias to "python".