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.