serialhdl: Clear "hupcl" bit from serial port

The arduino style serial port interfaces can reset the MCU when the
serial port is opened.  Clearing the HUPCL flag makes this less
likely.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/util.py

@@ -16,6 +16,12 @@ def set_nonblock(fd):
     fcntl.fcntl(fd, fcntl.F_SETFL
                 , fcntl.fcntl(fd, fcntl.F_GETFL) | os.O_NONBLOCK)
 
+# Clear HUPCL flag
+def clear_hupcl(fd):
+    attrs = termios.tcgetattr(fd)
+    attrs[2] = attrs[2] & ~termios.HUPCL
+    termios.tcsetattr(fd, termios.TCSADRAIN, attrs)
+
 # Support for creating a pseudo-tty for emulating a serial port
 def create_pty(ptyname):
     mfd, sfd = pty.openpty()