[pkg-go] Bug#886894: do not use daemon when systemd is available

[Antoine Beaupre]

Package: mtail
Version: 3.0.0~rc2-1
Severity: wishlist

daemon duplicates the functionality of "daemon". One shouldn't need to
install yet another "daemon" program if systemd is already installed
(and one is willing to use it of course).

Like prometheus-node-exporter, the dependency should be "daemon |
systemd-sysv" so that systemd-enabled systems do not need to install
the dependency.

There is the problem of what to do with the /etc/default/mtail
file. There are two options here:

 1. take the same route as prometheus-node-exporter and reuse the file
    as an EnvironmentFile. that will be messy because there are a lot
    of variables and it may make it difficult to port the
    file. furthermore, it will encounter similar issues there, like
    #886891

 2. just specify a sane set of default settings that can be overriden
    using normal systemd mechanisms, for example:

    https://askubuntu.com/a/659268

I believe it should be possible to replace the `daemon` dependency for
mtail without problems. The templating features of systemd can also
make it possible to easily start multiple instances of mtail, by the
way...

-- System Information:
Debian Release: 9.3
  APT prefers stable
  APT policy: (500, 'stable'), (1, 'experimental'), (1, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-5-amd64 (SMP w/4 CPU cores)
Locale: LANG=fr_CA.UTF-8, LC_CTYPE=fr_CA.UTF-8 (charmap=UTF-8), LANGUAGE=fr_CA.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages mtail depends on:
ii  adduser  3.115
pn  daemon   <none>
ii  libc6    2.24-11+deb9u1

mtail recommends no packages.

mtail suggests no packages.