Fix userauth so that it works together with SQLITE_OMIT_SHARED_CACHE.

([forum:/forumpost/0bfc5888a384d430|Forum post 0bfc5888a384d430]).
However, also change to code to issue a deprecation warning whenever
SQLITE_USER_AUTHENTICATION is used.

FossilOrigin-Name: 249048b0cbc37058c229a785182e07b46029584aff9c385269b0b93503153476

src/sqliteInt.h

@@ -1596,6 +1596,10 @@ struct FuncDefHash {
 };
 #define SQLITE_FUNC_HASH(C,L) (((C)+(L))%SQLITE_FUNC_HASH_SZ)
 
+#if defined(SQLITE_USER_AUTHENTICATION)
+# warning  "The SQLITE_USER_AUTHENTICATION extension is deprecated. \
+ See ext/userauth/user-auth.txt for details."
+#endif
 #ifdef SQLITE_USER_AUTHENTICATION
 /*
 ** Information held in the "sqlite3" database connection object and used