Closes #1895. Thanks to Franz.

Closes #1880. Thanks to J1EXA, richl, and EnneiteZer.

This occurred when reloading a persistence file and
`per_listener_settings true` is set and the client did not set a
username.

Closes #1891. Thanks to Mikkel Nepper-Christensen.

Signed-off-by: Franz Auernigg <f.auernigg@commend.com>

Signed-off-by: Franz Auernigg <f.auernigg@commend.com>

Closes #1881. Thanks to Arnav Singh.

This allows a big swath of ifdefs to be removed. It also means savings as the db var is not passed around all of the time, and makes it easier to remove the final broker call to mosquitto_time() call in packet_mosq.c. In one test this reduced the calls to mosquitto_time() from 295k to 48k.

This change means privileges are dropped before loading certificates, starting logging, creating the pid file etc. are carried out, so all of those actions must now be changed to ensure that the unprivileged user can carry them out.