Stop using GLOBAL Windows API function pointers.

The declarations in header files now use ordinary 'extern'. That means
I have to arrange to put definitions matching those declarations in
the appropriate modules; so I've made a macro DEFINE_WINDOWS_FUNCTION
which performs a definition matching a prior DECLARE_WINDOWS_FUNCTION
(and reusing the typedef made by the latter).

This applies not only to the batch of functions that were marked
GLOBAL in winstuff.h, but also the auxiliary sets marked
WINCAPI_GLOBAL and WINSECUR_GLOBAL in wincapi.h and winsecur.h
respectively.

windows/wincapi.c

@@ -9,9 +9,10 @@
 #include "putty.h"
 #include "ssh.h"
 
-#define WINCAPI_GLOBAL
 #include "wincapi.h"
 
+DEF_WINDOWS_FUNCTION(CryptProtectMemory);
+
 bool got_crypt(void)
 {
     static bool attempted = false;