Pass stderr of program run with "target remote |"

    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.