Use ui_file_as_string in execute_command_to_string

Use ui_file_as_string in execute_command_to_string

... and then return std::string and adjust all callers.

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

	* gdbcmd.h (execute_command_to_string): Now returns std::string.
	(lookup_struct_elt_type): Adjust to use std::string.
	* top.c (execute_command_to_string): Use ui_file_as_string and
	return std::string.
	* guile/guile.c (gdbscm_execute_gdb_command): Adjust to use
	std::string.
	* python/python.c (execute_gdb_command): Adjust to use
	std::string.