Use std::string in ui_out_table

Use std::string for the id field of the ui_out_table object.

I found that all users of ui_out_table_begin passed a non-NULL value to
the tblid parameter, so we don't have to worry about the NULL case.  I
changed the tblid parameter to be a std::string while at it.

gdb/ChangeLog:

	* ui-out.c (struct ui_out_table) <id>: Change type to
	std::string.
	(ui_out_table_begin): Change tblid parameter type to
	std::string, adapt code.
	update following type change.
	(clear_table): Update.
	(ui_out_new): Update.