RC1 on Native Win32 - patch required

Hello!

The following very short patch is required to make the native Win32 client
work with 7.1RC1. Without it, libpq works fine, but not psql.
Two things are patched:
1) mainloop.c, call to "pqsignal" is wrapped in #ifndef WIN32. All other
calls to pqsignal are already wrapped, so it shuold be safe.
2) config.h.win32 - added "#define HAVE_ATEXIT" since Win32 uses atexit()
and not on_exit().

I hope these can go in before release - it seems to me they are safe because
they affect only the native Win32 platform.

With this applied, client side on Win32 native can move to the "confirmed
working" list.

//Magnus

<<win32_patch_RC1.diff>>