cli: ignore EINTR caused by SIGWINCH when calling poll()

This can happen when adjusting a tmux pane running mininet cli.  The
process will receive SIGWINCH causing the poll() call to raise
select.error

Below is a session dump with strace

	09:55:52.498106 poll([{fd=0, events=POLLIN}, {fd=28, events=POLLIN}], 2, -1) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
	09:55:52.606734 --- SIGWINCH {si_signo=SIGWINCH, si_code=SI_KERNEL} ---
	09:55:52.606770 rt_sigreturn({mask=[]}) = -1 EINTR (Interrupted system call)
	Traceback (most recent call last):
	  File "examples/topofac.py", line 527, in <module>
	    run()
	  File "examples/topofac.py", line 522, in run
	    CLI( net )
	  File "/home/yunion/git-repo/mininet/mininet/cli.py", line 68, in __init__
	    self.run()
	  File "/home/yunion/git-repo/mininet/mininet/cli.py", line 103, in run
	    self.cmdloop()
	  File "/home/yunion/.usr/lib/python2.7/cmd.py", line 142, in cmdloop
	    stop = self.onecmd(line)
	  File "/home/yunion/.usr/lib/python2.7/cmd.py", line 220, in onecmd
	    return self.default(line)
	  File "/home/yunion/git-repo/mininet/mininet/cli.py", line 422, in default
	    self.waitForNode( node )
	  File "/home/yunion/git-repo/mininet/mininet/cli.py", line 440, in waitForNode
	    bothPoller.poll()
	select.error: (4, 'Interrupted system call')
	09:55:52.622076 rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7ff9a9b936d0}, {0x7ff9a9ef2230, [], SA_RESTORER, 0x7ff9a9b936d0}, 8) = 0
	09:55:52.631905 +++ exited with 1 +++

mininet/cli.py

@@ -29,6 +29,8 @@ from subprocess import call
 from cmd import Cmd
 from os import isatty
 from select import poll, POLLIN
+import select
+import errno
 import sys
 import time
 import os
@@ -459,6 +461,13 @@ class CLI( Cmd ):
                 # it's possible to interrupt ourselves after we've
                 # read data but before it has been printed.
                 node.sendInt()
+            except select.error as e:
+                # pylint: disable=unpacking-non-sequence
+                errno_, errmsg = e.args
+                # pylint: enable=unpacking-non-sequence
+                if errno_ != errno.EINTR:
+                    error( "select.error: %d, %s" % (errno_, errmsg) )
+                    node.sendInt()
 
     def precmd( self, line ):
         "allow for comments in the cli"