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