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