Replace flags argument to Sql.open with single bool

Previously, "flags" argument of Sql.open was of type libc::c_int, but
only one bit was used: whether to open database read-only. This commit
makes it explicit by changing type to bool and renaming argument.

src/context.rs

@@ -150,7 +150,7 @@ impl Context {
         };
 
         ensure!(
-            ctx.sql.open(&ctx, &ctx.dbfile, 0),
+            ctx.sql.open(&ctx, &ctx.dbfile, false),
             "Failed opening sqlite database"
         );