feat(fmt): support systemd journal priority prefixes

[sant0s12]

When JOURNAL_STREAM is set, log lines are prefixed with <N> (syslog priority) so journald picks up the correct severity.

API

use env_logger::{Builder, fmt::JournalPrefix};

let mut builder = Builder::new();
builder.format_journal_prefix(JournalPrefix::Auto);    // default
builder.format_journal_prefix(JournalPrefix::Enable);  // always prefix
builder.format_journal_prefix(JournalPrefix::Disable); // never prefix

• Auto (default): enables prefixes when JOURNAL_STREAM is set, resolved once at logger construction
• Enable: always prepend <N>
• Disable: never prepend

Priority mapping

Level	Prefix	Syslog
Error	<3>	LOG_ERR
Warn	<4>	LOG_WARNING
Info	<6>	LOG_INFO
Debug/Trace	<7>	LOG_DEBUG

Changes

• src/fmt/mod.rs: Added JournalPrefix enum, journal_prefix bool field on ConfigurableFormat, resolution in fmt::Builder::build(), and write_journal_prefix() in the formatter
• src/logger.rs: Added Builder::format_journal_prefix() method
• Made use log::Level unconditional (was behind #[cfg(feature = "color")])

[epage]

Note that we have an issue at #349 for this where the design is being worked through.

Independent of that, our contribution quality asks for people to work designs out in issues before posting PRs.

As this doesn't match an agreed to design, I'm going to close this for now.