Win32: GnuTLS and Unicode

GnuTLS does not support non-ANSI file paths on Win32. There are two ways to fix this. One is to load files manually and use the gnutls_setmem() API instead of gnutlsset_file (but it's pretty ugly if it's Win32 specific). Another is to bug the GnuTLS people to add Win32 specific file calls.