binding of /usr/lib/libgdbm_compat.so single file key-value database

## examples

check `examples/insert_update_query_delete.rs`

### open/close dbm

```rust
let db_filename = libc::tmpnam(std::ptr::null_mut());
let dbm_ptr = dbm_open(
    db_filename,
    libc::O_RDWR | libc::O_CREAT,
    libc::S_IRUSR | libc::S_IWUSR,
);
dbm_close(dbm_ptr);
```

### insert

```rust
let mut key = *b"black\0";
let key_datum = datum {
    dptr: key.as_mut_ptr().cast(),
    dsize: key.len() as i32 - 1,
};
let mut black = Color::new(0, 0, 0);
let value_datum = datum {
    dptr: (&mut black as *mut Color).cast(),
    dsize: Color::SIZE as i32,
};
dbm_store(dbm_ptr, key_datum, value_datum, StoreMode::DBM_INSERT);
```

### query

```rust
let value_datum = dbm_fetch(dbm_ptr, key_datum);
if !value_datum.dptr.is_null() {
    let mut color = std::mem::zeroed::<Color>();
    std::ptr::copy(
        value_datum.dptr.cast(),
        &mut color as *mut Color,
        value_datum.dsize as usize,
    );
    assert_eq!(color, black);
}
```

### delete

```rust
if dbm_delete(dbm_ptr, key_datum) == 0 {
    println!("delete success");
}
```