Use std::string for ui_out_hdr's text fields

Use std::string for ui_out_hdr's text fields

This patch makes ui_out_hdr use std::string for its text fields.  It
makes freeing automatic when the object is deleted.

gdb/ChangeLog:

	* mi/mi-out.c (mi_table_header): Change char * args to
	std::string.
	* cli-out.c (cli_table_header): Likewise.
	* ui-out.h (table_header_ftype): Likewise.
	(ui_out_table_header): Constify colhdr argument.
	(ui_out_query_field): Constify col_name argument.
	* ui-out.c (ui_out_hdr) <col_name, colhdr>: Change type to
	std::string.
	(uo_table_header): Change char * args to std::string.
	(ui_out_table_header): Likewise.
	(get_next_header): Constify colhdr argument and adapt.
	(clear_header_list): Don't free col_name/colhdr fields.
	(append_header_to_list): Change char * args to std::string and
	adapt.
	(verify_field): Constify variable.
	(ui_out_query_field): Constify col_name argument and adapt.
	* breakpoint.c (wrap_indent_at_field): Constify variable.