Make target_waitstatus_to_string return an std::string

Make target_waitstatus_to_string return an std::string

A quite straightforward change.  It does "fix" leaks in record-btrace.c,
although since this is only used in debug printing code, it has no real
world impact.

gdb/ChangeLog:

	* target/waitstatus.h (target_waitstatus_to_string): Change
	return type to std::string.
	* target/waitstatus.c (target_waitstatus_to_string): Return
	std::string.
	* target.h (target_waitstatus_to_string): Remove declaration.
	* infrun.c (resume, clear_proceed_status_thread,
	print_target_wait_results, do_target_wait, save_waitstatus,
	stop_all_threads): Adjust.
	* record-btrace.c (record_btrace_wait): Adjust.
	* target-debug.h
	(target_debug_print_struct_target_waitstatus_p): Adjust.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_wait_1): Adjust.
	* server.c (queue_stop_reply_callback): Adjust.