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/interprocess_mutex.c

@@ -13,8 +13,8 @@ HANDLE lock_interprocess_mutex(const char *mutexname, char **error)
     PACL acl = NULL;
     HANDLE mutex = NULL;
 
-    if (!make_private_security_descriptor(MUTEX_ALL_ACCESS,
-                                          &psd, &acl, error))
+    if (should_have_security() && !make_private_security_descriptor(
+            MUTEX_ALL_ACCESS, &psd, &acl, error))
         goto out;
 
     SECURITY_ATTRIBUTES sa;