improve systemd journald support

[jktjkt]

These three patches which make it easier to use systemd/journald's structured logging. When everything is in place, one can, for example, watch the log stream via this user-friendly :) command:

journlctl -f -u my-app.service \
  PRIORITY=1 PRIORITY=2 PRIORITY=3 PRIORITY=4 \
    SYSLOG_IDENTIFIER=only-important-messages-are-shown-now \
 + \
 PRIORITY=1 PRIORITY=2 PRIORITY=3 PRIORITY=4 PRIORITY=5 PRIORITY=6 PRIORITY=7 \
    SYSLOG_IDENTIFIER=I-want-details-from-this

That command will follow journal stream from the my-app.service. The logger whose name is only-important-messages-are-shown-now is rather heavily filtered, but everything from a logger called I-want-details-from-this is shown. These options are documented in journalctl's manpage. Sadly, there is neither range-checking nor negative matching in there.

I'm doing this mostly for use cases described in #1290 ...

journald: wider range of log severities

The previous code made it impossible to distinguish between the debug and trace severities. So instead of trying hard to preserve the equivalence of spdlog::level::debug with LOG_DEBUG, which is -- in the end -- just an arbitrary name, make sure that functionality is not lost.

Removed per explicit request; here's how to implement this:

#include <spdlog/sinks/systemd_sink.h>

template<typename Mutex>
class journald_sink : public spdlog::sinks::systemd_sink<Mutex> {
public:
    journald_sink()
    {
        this->syslog_levels_ = {/* spdlog::level::trace      */ LOG_DEBUG,
              /* spdlog::level::debug      */ LOG_INFO,
              /* spdlog::level::info       */ LOG_NOTICE,
              /* spdlog::level::warn       */ LOG_WARNING,
              /* spdlog::level::err        */ LOG_ERR,
              /* spdlog::level::critical   */ LOG_CRIT,
              /* spdlog::level::off        */ LOG_ALERT};
    }
};

journald: structured output for logger's name

Previously, the logger name was effectively lost. There were two choices on how to add it:

• Via a formatter, which would mean that journalctl would not be able to filter against that. That would be suboptimal.

• As a "syslog identifier". This means that journalctl will, by default, stop showing the daemon's executable name and replace that via the logger name. The PID is still shown, and if one would like to go back to the previous behavior, it is still possible via journalctl -o with-unit.

I think that the second option is strictly better than the first one.

fixes #1289

journald: fix source file location

This is what my manpage says, and what the original blog post says as well.

[gabime]

Thanks @jktjkt . Merged.