Use ifconfig for interface verification.

Previously we were using both ip link and ifconfig - not only is
this inconsistent and redundant, but it also broke when newer
ip link changed the reported names of certain interfacs to
"h1-eth0@36:".

Fixes #592

examples/hwintf.py

@@ -17,10 +17,11 @@ from mininet.util import quietRun
 
 def checkIntf( intf ):
     "Make sure intf exists and is not configured."
-    if ( ' %s:' % intf ) not in quietRun( 'ip link show' ):
+    config = quietRun( 'ifconfig %s 2>/dev/null' % intf, shell=True )
+    if not config:
         error( 'Error:', intf, 'does not exist!\n' )
         exit( 1 )
-    ips = re.findall( r'\d+\.\d+\.\d+\.\d+', quietRun( 'ifconfig ' + intf ) )
+    ips = re.findall( r'\d+\.\d+\.\d+\.\d+', config )
     if ips:
         error( 'Error:', intf, 'has an IP address,'
                'and is probably in use!\n' )