Class-ify lm_info_dsbt

Class-ify lm_info_dsbt

This patches makes lm_info_dsbt a "real" class.  It introduces a
destructor, initializes the field and replaces XCNEW/xfree with
new/delete.

gdb/ChangeLog:

	* solib-dsbt.c (struct lm_info_dsbt): Add destructor, initialize
	map field.
	(dsbt_current_sos): Allocate lm_info_dsbt with new.
	(dsbt_relocate_main_executable): Free lm_info_dsbt with delete
	and allocate with new.
	(dsbt_clear_solib, dsbt_free_so): Free lm_info_dsbt with delete.