journal: add StderrIsJournalStream function

To ensure that StderrIsJournalStream properly works in real conditions,
this test re-executes itself with systemd-run, and observes exit code
and logged entries.

A test that will be added in a latter commit requires systemd-run --wait,
which is not available on Ubuntu 16.04.

DEBIAN_FRONTEND=noninteractive prevents hang on "Setting up tzdata"
that became a problem on Ubuntu 20.04 (though something similar
could happen on any Debian/Ubuntu version).

See discussion in https://github.com/coreos/go-systemd/pull/410

internal/dlopen: fix test and run in CI

This adds one missing file that was forgotten during the initial
porting. Without that, the test could not even be built.
The internal package is now also properly tested by CI.

Source file is taken verbatim from https://github.com/coreos/pkg/tree/c7d1c02cb6cf99162b9cdbcad2a6a5c10af2ddca/dlopen.

This function can be used for automatic protocol upgrade described in
[1].

Both unit tests and runnable example are included. Only the latter
requires systemd, as unit tests are self-sufficient, and only test
that JOURNAL_STREAM environment variable is checked properly.

[1] https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading

login1: add methods to get session/user properties

login1: add context-aware ListSessions and ListUsers methods

workflows: add Go 1.19; update actions versions

Pin to the last patch release of a Go minor, rather than the first.

ci: add testing using 1.16/1.17/1.18

Go 1.17 introduced the new build constraint format[1] and `gofmt` now
complains if the new constraint format isn't present when the old
format is found.

[1] https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

journal: remove implicit initialization

some application could log to journald conditionally, the implicit
initialization will always create a socket on init.

We now create the socket only if necessary

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

dbus: add Connected methods to check connections status

dbus: add support for querying unit by PID

PID-based queries can now return either a name or a path

Allows users to get the unit name by PID. Similar to `systemctl status {pid}`

workflows: limit permissions to reading repo contents

dbus: implement support for cgroup freezer APIs

CONTRIBUTING: drop obsolete mailing list and IRC channel

*: rename master branch to main

These docs are long gone.

dbus: fix deprecated marks and docs

1. Fix "Deprecated:" annocations. They must be at the start of a new
   paragraph (IOW it needs an empty line before), otherwise tools
   (linters etc.) don't see them.

2. It is weird that the deprecated functions are documented, while
   the non-deprecated ones just refer to the deprecated ones. Move the
   documentation to non-deprecated functions.

3. Address some nits in docs:
   - add missing documentation for some methods;
   - add missing punctuation;
   - fix bad formatting (in particular, markdown is not supported);
   - minor rewording in a few places.

Fixes: 7172a8db ("dbus: add context-aware methods")
Fixes: c51f0924 ("dbus: add context to open functions")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

journal: close unix socket after successful probe

This is done to ensure unix sockets aren't left dangling after each Enabled() call, which
can lead to eventual process resource exhaustion. Fixes #366.