Better document how to listen on the network

GPSD does not listen to other addresses, i.e. it does not serve clients on network, even though -G is defined in /etc/default/gpsd.

Netstat output is as follows:

user@mydevice:~ $ netstat -l | grep gps
tcp 0 0 localhost:gpsd 0.0.0.0:* LISTEN
tcp6 0 0 localhost:gpsd [::]:* LISTEN
unix 2 [ ACC ] STREAM LISTENING 21487 /var/run/gpsd.sock

And ps -ax shows as follows:

3890 ? S<s 0:00 /usr/sbin/gpsd -G /dev/ttyUSB0

If GPSD is started from command line, then it serves network clients fine. Here is an example:

user@mydevice:~$ sudo -u gpsd /usr/sbin/gpsd -G /dev/ttyUSB0
user@mydevice:~$ netstat -l | grep gps
tcp 0 0 0.0.0.0:gpsd 0.0.0.0:* LISTEN
tcp6 0 0 [::]:gpsd [::]:* LISTEN

My assumption, that this is due to systemd overriding the -G option, as following is defined in /lib/systemd/system/gpsd.socket:

[Socket]
ListenStream=/var/run/gpsd.sock
ListenStream=[::1]:2947
ListenStream=127.0.0.1:2947

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu
Ubuntu 18.04.4 LTS

2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center
gpsd:
  Installed: 3.17-5
  Candidate: 3.17-5
  Version table:
 *** 3.17-5 500
        500 http://fi.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gpsd 3.17-5
ProcVersionSignature: Ubuntu 4.15.0-76.86-generic 4.15.18
Uname: Linux 4.15.0-76-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
Date: Mon Feb 10 09:18:29 2020
InstallationDate: Installed on 2018-11-12 (454 days ago)
InstallationMedia: Lubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: gpsd
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.gpsd: 2020-02-10T09:15:45.902653