gdb: const-ify unpack_* functions in remote.c

gdb: const-ify unpack_* functions in remote.c

Const-ify the unpack_* functions, and then adjust the callers
accordingly.

gdb/ChangeLog:

	* remote.c (class remote_target)
	<remote_unpack_thread_info_response,
	parse_threadlist_response>: Constify parameter and/or return
	value and or local variable.
	(stub_unpack_int): Likewise.
	(unpack_nibble): Likewise.
	(unpack_byte): Likewise.
	(unpack_int): Likewise.
	(unpack_string): Likewise.
	(unpack_threadid): Likewise.
	(remote_target::remote_unpack_thread_info_response): Likewise.
	(remote_target::parse_threadlist_response): Likewise.

Change-Id: Ibda75f664d6e3452df00f85af7134533049171b7