scripts: refactored daemon-stack.sh (#2496)

* refactored `daemon-stack.sh`

A new method was introduced to uniformaly start daemons and log output
accordingly. The methods for daemon start were renamed (plural ->
singular), therefore the adjustments in `start-mailserver.sh`.

* cleaned Fetchmail setup from `daemon-stack.sh`

Not sure why, but the Fetchmail setup was somehow happening in
`daemon-stack.sh` - this is not supposed to be the case. I relocated the
setup into `setup-stack.sh`, where it belong.

* delete old, unnecessary script in `target/bin/`

These are unused leftovers from the last commit, that relocated the
setup of Fetchmail into `setup.stack.sh`.

* corrected changedetector function name

* Apply suggestions from code review

* adjusted `debug-fetchmail` script

It is absolutely fine to source `setup-stack.sh` because sourcing the
script does not execute a single function (by desing of the script).
This way, we retain functionality.

* praise be ShellCheck

* added `log.sh` to `debug-fetchmail` as a dependency

* final cleanup

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>
Co-authored-by: Casper <casperklein@users.noreply.github.com>

target/bin/debug-fetchmail

@@ -1,6 +1,11 @@
 #! /bin/bash
 
-/usr/local/bin/setup-fetchmail
+# shellcheck source=../scripts/helpers/log.sh
+source /usr/local/bin/helpers/log.sh
+# shellcheck source=../scripts/startup/setup-stack.sh
+source /usr/local/bin/setup-stack.sh
+
+_setup_fetchmail
 
 su -s /bin/sh -c "/usr/bin/fetchmail \
   --verbose \