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-->
<!--end release-notes-assistant-->

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>

modules/log/level.go

@@ -39,6 +39,22 @@ var toString = map[Level]string{
 	NONE:  "none",
 }
 
+// Machine-readable log level prefixes as defined in sd-daemon(3).
+//
+// "If a systemd service definition file is configured with StandardError=journal
+// or StandardError=kmsg (and similar with StandardOutput=), these prefixes can
+// be used to encode a log level in lines printed. <...> To use these prefixes
+// simply prefix every line with one of these strings. A line that is not prefixed
+// will be logged at the default log level SD_INFO."
+var toJournalPrefix = map[Level]string{
+	TRACE: "<7>", // SD_DEBUG
+	DEBUG: "<6>", // SD_INFO
+	INFO:  "<5>", // SD_NOTICE
+	WARN:  "<4>", // SD_WARNING
+	ERROR: "<3>", // SD_ERR
+	FATAL: "<2>", // SD_CRIT
+}
+
 var toLevel = map[string]Level{
 	"undefined": UNDEFINED,
 
@@ -71,6 +87,10 @@ func (l Level) String() string {
 	return "info"
 }
 
+func (l Level) JournalPrefix() string {
+	return toJournalPrefix[l]
+}
+
 func (l Level) ColorAttributes() []ColorAttribute {
 	color, ok := levelToColor[l]
 	if ok {