Do not check if the bookmark title is a valid filename,
as we said in the user manual.
However, we should ensure that the string does not contain
NUL characters, since we assume that a valid bookmark storage
should not contain bookmarks with such names.
- Make sure all tables and indices are created for the database,
even the ones that are not used by BookmarkFS.
- Maintain the schema version in `PRAGMA user_version`.
- Always use `INT` for integer type, and `TEXT` for text type,
so that we could save a little space. This does not affect the
actual datatype (more precisely, type affinity) of the columns.
- Allow fdatasync(), since it is used by SQLite when commiting.
- Move `PRAGMA quick_check` to backend_create(), since it sometimes
calls stat() and cannot be sandboxed.
