libnftables1-sys 1.0.0

# Patch Analysis: JSON Output Support

## Email Request Summary

The contributor from the "reaction" project (a fail2ban alternative) requests adding JSON output support. They need to set the `NFT_CTX_OUTPUT_JSON` flag but cannot access it because `ctx` is private to the `Nftables` struct.

## Original Proposed Patch

```rust
pub fn set_json(&mut self) {
    self.set_flags(1 << 4);
}
```

## Recommendation: **ACCEPT** (with improvements)

### Risk Assessment: **VERY LOW**

The patch is safe and follows existing patterns perfectly.

## Improvements Applied

Instead of accepting the patch as-is with hardcoded bit values, I've made the codebase **more maintainable** by using named constants from the libnftables header.

### Changes Made

#### 1. Updated `build.rs` (line 71)
**Added**: `.allowlist_var("NFT_.*")` to bindgen configuration

**Why**: This tells bindgen to generate Rust constants for all C constants starting with `NFT_`, including:
- `NFT_CTX_OUTPUT_HANDLE` (bit 3)
- `NFT_CTX_OUTPUT_JSON` (bit 4)  
- `NFT_CTX_OUTPUT_NUMERIC_TIME` (bit 10)
- All other NFT_CTX_OUTPUT_* flags

These constants are defined in `/usr/include/nftables/libnftables.h` lines 58-75.

#### 2. Updated `src/lib.rs`
**Changed three methods** to use named constants instead of magic numbers:

**Before**:
```rust
pub fn set_output_handle(&mut self) {
    self.set_flags(1 << 3);
}

pub fn set_numeric_time(&mut self) {
    self.set_flags(1 << 10);
}
```

**After**:
```rust
pub fn set_output_handle(&mut self) {
    self.set_flags(NFT_CTX_OUTPUT_HANDLE);
}

pub fn set_numeric_time(&mut self) {
    self.set_flags(NFT_CTX_OUTPUT_NUMERIC_TIME);
}

pub fn set_json(&mut self) {
    self.set_flags(NFT_CTX_OUTPUT_JSON);
}
```

## Benefits

### 1. **Maintainability**
- No more magic numbers (`1 << 3`, `1 << 4`, `1 << 10`)
- Self-documenting code
- If libnftables changes flag values (unlikely but possible), a rebuild picks up the new values automatically

### 2. **Correctness**
- Constants come directly from libnftables header
- No risk of typos in bit positions
- Compiler will error if constant doesn't exist

### 3. **Consistency**
- All three flag setter methods now use the same pattern
- Follows Rust FFI best practices
- Matches approach used by other libnftables-sys implementations (see chifflier/libnftables-sys PR #1)

### 4. **Discoverability**
- Other `NFT_CTX_OUTPUT_*` constants now available for future use:
  - `NFT_CTX_OUTPUT_REVERSEDNS`
  - `NFT_CTX_OUTPUT_SERVICE`
  - `NFT_CTX_OUTPUT_STATELESS`
  - `NFT_CTX_OUTPUT_ECHO`
  - `NFT_CTX_OUTPUT_GUID`
  - `NFT_CTX_OUTPUT_NUMERIC_PROTO`
  - `NFT_CTX_OUTPUT_NUMERIC_PRIO`
  - `NFT_CTX_OUTPUT_NUMERIC_SYMBOL`
  - `NFT_CTX_OUTPUT_NUMERIC_ALL`
  - `NFT_CTX_OUTPUT_TERSE`

## No Breaking Changes

- All changes are **additive** or **internal improvements**
- Existing API unchanged (except now uses constants internally)
- Builds should still work identically
- Tests should still pass

## Historical Context

This fix was previously applied in chifflier/libnftables-sys PR #1 (January 2022) when updating for nftables 1.0.0 compatibility. The PR added:
1. `.allowlist_var("NFT_CTX_.*")` to build.rs
2. Use of `NFT_CTX_OUTPUT_JSON` constant instead of hardcoded values

We're applying the same best practice here.

## Testing

After these changes:
1. Clean build should succeed: `cargo clean && cargo build`
2. Tests should pass (with root): `sudo -E cargo test`
3. Generated bindings will include NFT_CTX_OUTPUT_* constants

## Response to Contributor

**Suggested email response**:

```
Thank you for the patch and for using libnftables1-sys in your project!

I've accepted your request to add JSON support, but made a small improvement:
instead of hardcoding bit values (1 << 4), the code now uses the named
constant NFT_CTX_OUTPUT_JSON from the libnftables header.

I also updated the existing set_output_handle() and set_numeric_time()
methods to use named constants (NFT_CTX_OUTPUT_HANDLE and
NFT_CTX_OUTPUT_NUMERIC_TIME) for consistency.

The new method is:

    pub fn set_json(&mut self) {
        self.set_flags(NFT_CTX_OUTPUT_JSON);
    }

This makes the code more maintainable and self-documenting. The build.rs
now includes `.allowlist_var("NFT_.*")` so all NFT_CTX_OUTPUT_* constants
are available.

The changes are committed and will be in the next release.
```

## References

- libnftables header: `/usr/include/nftables/libnftables.h`
- Original implementation: https://github.com/chifflier/libnftables-sys/pull/1
- NFT_CTX_OUTPUT flags defined at: libnftables.h:58-75