[Debugging output] Make remote packet truncation length adjustable

[Debugging output] Make remote packet truncation length  adjustable

While debugging, i felt the need to adjust the truncation length of remote
packets so i could see more or less data as needed. The default is currently
set to 512 bytes.

This patch makes this option adjustable through the new "set debug
remote-packet-max-chars" command. It can be set to unlimited if we want to
completely disable truncation.

Update on v5:

- Adjusted function and variable documentation, NEWS entry and GDB manual.

gdb/ChangeLog:

2019-11-25  Luis Machado  <luis.machado@linaro.org>

	* NEWS (New Commands): Mention "set debug remote-packet-max-chars".
	* remote.c (REMOTE_DEBUG_MAX_CHAR): Remove.
	(remote_packet_max_chars): New static global.
	(show_remote_packet_max_chars): New function.
	(remote_target::putpkt_binary): Adjust to use new
	remote_packet_max_chars option.
	(remote_target::getpkt_or_notif_sane_1): Likewise.
	(_initialize_remote): Register new remote-packet-max-chars option.

gdb/doc/ChangeLog:

2019-11-25  Luis Machado  <luis.machado@linaro.org>

	* gdb.texinfo (Debugging Output): Document set debug
	remote-packet-max-chars.

Change-Id: I2e871b37bfcaa6376537c3fe3db8f016dd806a7c