w32: don't disable the error reporting dialog

Windows users expect this when a program crashes. Without it, the
program just disappears. Also change the SetErrorMode call to use macros
instead of a hardcoded constant.
w32: enable heap corruption detection

Enable the terminate-on-corruption feature. This is recommended for new
Windows applications and shouldn't cause a performance hit. It actually
shouldn't change anything for 64-bit builds, since Win64 has this
switched on by default.

See:
http://blogs.msdn.com/b/michael_howard/archive/2008/02/18/faq-about-heapsetinformation-in-windows-vista-and-heap-based-buffer-overruns.aspx
w32: use safe DLL search paths everywhere

Windows applications that use LoadLibrary are vulnerable to DLL
preloading attacks if a malicious DLL with the same name as a system DLL
is placed in the current directory. mpv had some code to avoid this in
ao_wasapi.c. This commit just moves it to main.c, since there's no
reason it can't be used process-wide.

This change can affect how plugins are loaded in AviSynth, but it
shouldn't be a problem since MPC-HC also does this and it's a very
popular AviSynth client.