Fix Win32 portability problems

Fix Win32 portability problems

The network filter / snapshot / hooks code introduced some
non-portable pices that broke the win32 build

* configure.ac: Check for net/ethernet.h required by nwfile config
   parsing code
* src/conf/nwfilter_conf.c: Define ethernet protocol  constants
  if net/ethernet.h is missing
* src/util/hooks.c: Disable hooks build on Win32 since it lacks
  fork/exec/pipe
* src/util/threads-win32.c: Fix unchecked return value
* tools/virsh.c: Disable SIGPIPE on Win32 since it doesn't exist.
  Fix non-portable strftime() formats