[louisabraham]

Adds Conn.SetDefensive(enabled bool) error, which calls sqlite3_db_config(db, SQLITE_DBCONFIG_DEFENSIVE, …) through the existing db_config_onoff helper.

While the flag is on, SQL cannot deliberately corrupt the database file: PRAGMA writable_schema=ON, PRAGMA journal_mode=OFF, writes to sqlite_dbpage and direct writes to shadow tables are turned off. See https://www.sqlite.org/c3ref/c_dbconfig_defensive.html#sqlitedbconfigdefensive.

The name and signature are the same as (*sqlite.Conn).SetDefensive in zombiezen.com/go/sqlite, so code written against that package keeps building here.

Context: we run untrusted SQL in a sandboxed service and are moving it from zombiezen.com/go/sqlite to this package to get the C build of SQLite. Defensive mode is the only setting we use that has no equivalent here.

Tests

• TestSetDefensive creates a table, sets writable_schema = ON, then writes to sqlite_schema. The write fails with the flag on and succeeds with it off. The test also fails if SetDefensive is changed to do nothing.
• go test ./... passes.
• CHANGELOG.md has an entry under Unreleased.