As discussed in PR slog-rs#5, this is based on the [syslog backend that I wrote for sloggers][1] and does not use the `syslog` crate. It is now very different from both slog-syslog 0.12 and PR slog-rs#5. Differences from slog-rs#5 include:

* Uses the POSIX syslog API through the `libc` crate. (See “design and rationale” in src/lib.rs, starting at line 52, for details.) This means:
* Only works on Unix-like platforms.
* Only sends log messages to the local syslog daemon, not to a remote server.
* Formats log messages into a reusable thread-local buffer, as in slog-syslog 0.12.
* The `Drain` implementation is now called `SyslogDrain` instead of `Streamer3164`, since local syslog doesn't use the RFC3164 protocol.
* If the `serde` feature is enabled, logging settings can be loaded from a configuration file.

Minimum supported Rust version is 1.27.2, or if the `serde` feature is enabled, 1.31.0. Doc-tests require 1.28.0. Works (except doc-tests) on 1.26.2 if you run the following commands first:

```
cargo generate-lockfile
cargo update --package lazy_static --precise 1.3.0
cargo update --package serde --precise 1.0.58
cargo update --package serde_derive --precise 1.0.58
```

[1]: sile/sloggers#34

… a given syslog priority

…n a per-message basis.

Thanks to @eun-ice for the idea and initial implementation.