Manually watch and parse journal log files

[dead-claudia]

This will bring a nice perf boost as I can decode a lot less, and it'll kick to the curb one of two reasons I interface with libsystemd at all. (The other, sd_notify, is very easy to reimplement.)

I can then make a few new extremely-useful changes:

• I can count all fields without needing to even inspect any of them. (The reason why I'm not listing fields right now is because I don't want the allocating and copying nonsense that sd_journal_* field iterators currently do.)
• I can completely drop the libsystemd dependency. This simplifies building by a lot, and I don't have to worry as hard about linking.
  • It also lets me use musl libc and make binaries entirely self-contained.
• I can report generic per-field lengths and not just message lengths and such.
• I can do streaming decompression of large entries.
  • I can reuse a shared thread-local buffer for everything, making it zero-copy at runtime.
  • Message filters (Add message filter support #19) can use streaming regexps, though that'll unfortunately require me to roll my own regexp implementation (the POSIX API offers no incremental processing capability).
  • Does mean I have to take on three dependencies: lz4, xz, and zstd.
• I can report the true size of all ingested journal data, in both disk size and actual processed size.
• I can track all message fields and not just the first. (Q: multiline support? #23 is an example of how this could matter.)

Useful links:

• sd_notify: https://www.freedesktop.org/software/systemd/man/devel/sd_notify.html#Notes
• journald file format: https://systemd.io/JOURNAL_FILE_FORMAT/ (This is documented as fully stable)
• journald structure: https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html#Files

Note: This requires inotify and file reading. Be sure to only perform one inotify read at a time. Also, inotify only detects FS modifications - epoll is still required for detecting readability, and so I'll have to use both in concert.