Make target_options_to_string return an std::string

Return an std::string instead of a char *, saving some manual freeing.

I only manually tested with "set debug target 1" and "set debug lin-lwp
1", since this only deals with debug output.

gdb/ChangeLog:

	* target.h (target_options_to_string): Return an std::string.
	* target.c (str_comma_list_concat_elem): Return void, use
	std::string.
	(do_option): Likewise.
	(target_options_to_string): Return an std::string.
	* linux-nat.c (linux_nat_target::wait): Adjust.
	* target-debug.h (target_debug_print_options): Adjust.