Use ui_file_as_string in gdb/python/

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* python/py-arch.c (archpy_disassemble): Use ui_file_as_string and
	std::string.
	* python/py-breakpoint.c (bppy_get_commands): Use
	ui_file_as_string and std::string.
	* python/py-frame.c (frapy_str): Likewise.
	* python/py-type.c (typy_str): Likewise.
	* python/py-unwind.c (unwind_infopy_str): Likewise.
	* python/py-value.c (valpy_str): Likewise.