Add stdio support to gdbreplay

I've been using gdbreplay to help debug an intermittent failure, and I
wanted it to be a little simpler to use.  This patch adds support for
"-" as the "address" argument.  With this patch you can do:

    (gdb) target remote | gdbreplay logfile -

... and not have to start gdbreplay in a separate shell.

2021-02-12  Tom Tromey  <tromey@adacore.com>

	* gdbreplay.cc (remote_desc): Remove.
	(remote_desc_in, remote_desc_out): New globals.
	(remote_close): Update.
	(remote_open): Handle "-".
	(remote_open): Update.
	(logchar): Log to stderr.
	(expect, play): Update.