Make get_user_sid() return the cached copy if one already exists.

A user reported in January that locking down our process ACL causes
get_user_sid's call to OpenProcessToken to fail with a permissions
error. This _shouldn't_ be important, because we'll already have found
and cached the user SID before getting that far - but unfortunately
the call to get_user_sid in winnpc.c was bypassing the cache and
trying the whole process again.

This fix changes the memory ownership semantics of get_user_sid():
it's now an error to free the value it gives you, or else the *next*
call to get_user_sid() will return a stale pointer. Hence, also
removed those frees everywhere they appear.

windows/winsecur.c

@@ -44,6 +44,9 @@ PSID get_user_sid(void)
     DWORD toklen, sidlen;
     PSID sid = NULL, ret = NULL;
 
+    if (usersid)
+        return usersid;
+
     if (!got_advapi())
         goto cleanup;
 
@@ -73,7 +76,7 @@ PSID get_user_sid(void)
 
     /* Success. Move sid into the return value slot, and null it out
      * to stop the cleanup code freeing it. */
-    ret = sid;
+    ret = usersid = sid;
     sid = NULL;
 
   cleanup: