Add the sqlite3_db_readonly() interface. This is still tentative, pending

a closer look at other ideas to accomplish the same thing.

FossilOrigin-Name: 254f99ea9ff1534948bdb179e69ab0c940c87ec1

src/sqliteInt.h

@@ -2701,6 +2701,7 @@ void sqlite3AddCollateType(Parse*, Token*);
 void sqlite3EndTable(Parse*,Token*,Token*,Select*);
 int sqlite3ParseUri(const char*,const char*,unsigned int*,
                     sqlite3_vfs**,char**,char **);
+Btree *sqlite3DbNameToBtree(sqlite3*,const char*);
 int sqlite3CodeOnce(Parse *);
 
 Bitvec *sqlite3BitvecCreate(u32);