Don't print too much if remote_debug is on

Don't print too much if remote_debug is on

If we turn "remote debug" on and GDB does some vFile operations,
a lot of things will be printed in the screen, which makes
"remote debug" useless.

This patch changes the code that we only print 512 chars in max in
debugging messages, like this,

Sending packet: $qXfer:features:read:target.xml:0,fff#7d...Packet received: l<?xml version="1.0"?>\n<!-- Copyright (C) 2010-2016 Free Software Foundation, Inc.\n\n     Copying and distribution of this file, with or without modification,\n     are permitted in any medium without royalty provided the copyright\n     notice and this notice are preserved.  -->\n\n<!-- AMD64 with AVX - Includes Linux-only special "register".  -->\n\n<!DOCTYPE target SYSTEM "gdb-target.dtd">\n<target>\n  <architecture>i386:x86-64</architecture>\n  <osabi>GNU/Linux</osabi>\n  <xi:include href="64bit-core.xml"/>\n  <xi:[14 bytes omitted]

Sending packet: $qXfer:auxv:read::0,1000#6b...Packet received: l!\000\000\000\000\000\000\000\000d\000\000\000\000\000\000\000\003\000\000\000\000\000\000\000@\000@\000\000\000\000\000\004\000\000\000\000\000\000\0008\000\000\000\000\000\000\000\005\000\000\000\000\000\000\000\t\000\000\000\000\000\000\000\a\000\000\000\000\000\000\000\177\000\000\b\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\t\000\000\000\000\000\000\000\000\004@\000\000\000\000\000\013\000\000\000\000\000\000\003\000\000\000\000\000\000\f\000\000\000\000\000\000\003\000\000\000\000\000\000\r\000\000\000\000\000\000\003\000\000\000\000\000\000\016\000\000\000\000\000\000\003\000\000\000\000\000\000\027\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\031\000\000\000\000\000\000\177\000\000\037\000\000\000\000\000\000\000\000\017\000\000\000\000\000\000\00\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000[582 bytes omitted]

gdb:

2017-01-13  Yao Qi  <yao.qi@linaro.org>

	* remote.c (REMOTE_DEBUG_MAX_CHAR): New macro.
	(putpkt_binary): Print only REMOTE_DEBUG_MAX_CHAR chars in debug
	output.
	(getpkt_or_notif_sane_1): Likewise.