log: journald integration (#2869)

Provide a bit more journald integration. Specifically:

- support emission of printk-style log level prefixes, documented in [`sd-daemon`(3)](https://man7.org/linux/man-pages/man3/sd-daemon.3.html#DESCRIPTION), that allow journald to automatically annotate stderr log lines with their level;
- add a new "journaldflags" item that is supposed to be used in place of "stdflags" when under journald to reduce log clutter (i. e. strip date/time info to avoid duplication, and use log level prefixes instead of textual log levels);
- detect whether stderr and/or stdout are attached to journald by parsing `$JOURNAL_STREAM` environment variable and adjust console logger defaults accordingly.

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## Draft release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/2869): <!--number 2869 --><!--line 0 --><!--description bG9nOiBqb3VybmFsZCBpbnRlZ3JhdGlvbg==-->log: journald integration<!--description-->
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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/2869
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Ivan Shapovalov <intelfx@intelfx.name>
Co-committed-by: Ivan Shapovalov <intelfx@intelfx.name>

custom/conf/app.example.ini

@@ -635,7 +635,7 @@ LEVEL = Info
 ;[log.%(WriterMode)]
 ;MODE=console/file/conn/...
 ;LEVEL=
-;FLAGS = stdflags
+;FLAGS = stdflags or journald
 ;EXPRESSION =
 ;PREFIX =
 ;COLORIZE = false