Windows Pageant: turn 'has_security' into a global function.

Now it can be called from places other than Pageant's WinMain(). In
particular, the attempt to make a security descriptor in
lock_interprocess_mutex() is gated on it.

In return, however, I've tightened up the semantics. In normal PuTTY
builds that aren't trying to support pre-NT systems, the function
*unconditionally* returns true, on the grounds that we don't expect to
target any system that doesn't support the security APIs, and if
someone manages to contrive one anyway - or, more likely, if we some
day introduce a bug in our loading of the security API functions -
then this safety catch should make Pageant less likely to accidentally
fall back to 'never mind, just run in insecure mode'.

windows/utils/security.c

@@ -20,6 +20,20 @@ DEF_WINDOWS_FUNCTION(GetSecurityInfo);
 DEF_WINDOWS_FUNCTION(SetSecurityInfo);
 DEF_WINDOWS_FUNCTION(SetEntriesInAclA);
 
+bool should_have_security(void)
+{
+#ifdef LEGACY_WINDOWS
+    /* Legacy pre-NT platforms are not expected to have any of these APIs */
+    init_winver();
+    return (osPlatformId == VER_PLATFORM_WIN32_NT);
+#else
+    /* In the up-to-date PuTTY builds which do not support those
+     * platforms, unconditionally return true, to minimise the risk of
+     * compiling out security checks. */
+    return true;
+#endif
+}
+
 bool got_advapi(void)
 {
     static bool attempted = false;