Avoid some manual memory management in Python

Avoid some manual memory management in Python

This changes a few places in the Python code to avoid manual memory
management, in favor of letting std::string do the work.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* python/python.c (compute_python_string): Return std::string.
	(gdbpy_eval_from_control_command): Update.
	(do_start_initialization): Use std::string.
	* python/py-varobj.c (py_varobj_iter_next): Use string_printf, not
	xstrprintf.
	* python/py-breakpoint.c (local_setattro): Use string_printf, not
	xstrprintf.