After the tty is opened, the fd is passed off to a GIOChannel which
manages it. When the serial port is closed with _close_internal(),
g_io_channel_shutdown() is called which then calls g_io_unix_close() which
in turn close()s the fd.
_close_internal() performs a second explicit close() after the
g_io_channel_shutdown() which *in the best case scenario* will fail with
EBADF.
This is a race condition that can result in other file descriptors being
closed.
This change avoids double closing the serial port fd -- the shutdown() call
is removed in favour of the explicit close().
