Pass stderr of program run with "target remote |"
        via gdb_stderr.
	* serial.c (serial_open): Set error_fd to -1.
	* serial.h (struct serial): New field error_fd.
	(struct serial_opts): New field avail.
	* ser-pipe.c (pipe_open): Create another pair
	of sockets.  Pass stderr to gdb.
	* ser-mingw.c (pipe_windows_open): Pass
	PEX_STDERR_TO_PIPE to pex_run.  Initialize
	sd->error_fd.
	(pipe_avail): New.
	(_initialize_ser_windows): Hook pipe_avail.
	* ser-base.c (generic_readchar): Check if there's
	anything in stderr channel and route that to gdb_stderr.