Add the ".user" shell command and implement the sqlite3_user_add()

routine.  Incremental check-in.  The code compiles but does not work.

FossilOrigin-Name: a0455f9deb603bf91684158d911269622720fc1a

src/prepare.c

@@ -722,7 +722,7 @@ static int sqlite3LockAndPrepare(
       db->auth.authLevel = authLevel;
     }
     if( db->auth.authLevel<UAUTH_User ){
-      sqlite3ErrorWithMsg(db, SQLITE_ERROR, "user not authenticated");
+      sqlite3ErrorWithMsg(db, SQLITE_AUTH_USER, "user not authenticated");
       sqlite3_mutex_leave(db->mutex);
       return SQLITE_ERROR;
     }