Link and build tor-fw-helper on Windows

- Update configure script to test for libminiupnpc along with the
  libws2_32 and libiphlpapi libraries required by libminiupnpc
- When building tor-fw-helper, link in libiphlpapi
- Link in libminiupnpc statically becasue I could not get the DLL
  to link properly
- Call WSAStartup before doing network operations
- Fix up a compiler warning about uninitialized backend_state

N.B. The changes to configure.in and Makefile.am will break on non-
Windows platforms.