Fix commit r2386-2387 for real: * configure checks that MX-wire-compat is disabled instead of   just randomly looking at the MTU when enforcing packet sizes * only display the packet size message and value in configure   when they are actually modified * simplify things and fixup the mess
Fix commit r2386-2387 for real:
* configure checks that MX-wire-compat is disabled instead of
  just randomly looking at the MTU when enforcing packet sizes
* only display the packet size message and value in configure
  when they are actually modified
* simplify things and fixup the mess

git-svn-id: 29c1264a5cf5e3532df57b06678e7347571c1a3c@2407 f1ba3bf5-cb5c-402b-92a9-7c6bdc83a356

ananos/xen2mx,ananos/xen2mx,ananos/open-mx,ananos/open-mx,ananos/xen2mx,ananos/open-mx,ananos/xen2mx