running the service in a sandbox. read-only root file system,
with tmpfs mounted in /tmp, hidden /root and /home,
temporary /dev. the only writeable path is the data directory,
which according to my experiments is enough for the service
to work correctly.
If you have more than 1 User with hasedPassword Option set it generates
```
rm -f /var/lib/mosquitto/passwd
touch /var/lib/mosquitto/passwd
echo 'user1:$6$xxx' > /var/lib/mosquitto/passwd
echo 'user2:$6$xxx' > /var/lib/mosquitto/passwd
```
Which ends up in only having 1 user.
Fixes https://github.com/NixOS/nixpkgs/issues/27996.
Updates instructions for generating hashes passwords for use in a
Mosquitto password file. Using `mosquitto_passwd` to generate these
hashes is a little less convenient, but the results are more likely to
be compatible with the mosquitto daemon.
As far as I can tell, the hashes generated with `mkpassd` did not work
as intended. But this may have been hidden by another bug:
https://github.com/NixOS/nixpkgs/issues/27130.
Related to https://github.com/NixOS/nixpkgs/issues/27130.
Adds an option to NixOS configuration option to have Mosquitto use the
password file that it generates. When this option is false the
Mosquitto server will accept login attempts with any username and any
password. This option defaults to false because this matches the
behavior of the service prior to the introduction of this option.
When the `services.mosquitto.checkPasswords` is true, the server will
only accept valid usernames and passwords.
