Make program_space::deleted_solibs a vector of std::string

This allows removing a usage of free_char_ptr_vec.

gdb/ChangeLog:

	* progspace.h (struct program_space) <deleted_solibs>: Change
	type to std::vector<std::string>.
	* progspace.c (clear_program_space_solib_cache): Adjust.
	* breakpoint.c (print_solib_event): Adjust.
	(check_status_catch_solib): Adjust.
	* solib.c (update_solib_list): Adjust.
	* ui-out.h (class ui_out) <field_string>: New overload.
	* ui-out.c (ui_out::field_string): New overload.