Class-ify lm_info_target

Class-ify lm_info_target

This patch makes lm_info_target a "real" class.  It adds a destructor,
uses std::string, initializes the fields and replaces XCNEW/xfree with
new/delete.

gdb/ChangeLog:

	* solib-target.c (struct lm_info_target): Add destructor,
	initialize fields.
	<name>: Change type to std::string.
	(library_list_start_library): Allocate lm_info_target with new.
	(solib_target_free_library_list): Free lm_info_target with
	delete.
	(solib_target_current_sos): Adapt to std::string.
	(solib_target_free_so): Free lm_info_target with delete.