Complete Refactor for target/bin (#1654)

* documentation and script updates trying to fix #1647
* preparations for refactoring target/bin/
* complete refactor for target/bin/
* changing script output slightly
* outsourcing functions in `bin-helper.sh`
* re-wrote linting to allow for proper shellcheck -x execution
* show explanation for shellcheck ignore
* adding some more information

target/bin/debug-fetchmail

@@ -1,11 +1,11 @@
-#! /bin/sh
+#! /bin/bash
 
 /usr/local/bin/setup-fetchmail
 
 su -s /bin/sh -c "/usr/bin/fetchmail \
-	--verbose \
-	--daemon 0 \
-	--check \
-	--nosyslog \
-	--nodetach \
-	-f /etc/fetchmailrc" fetchmail <&- 2>&1
+  --verbose \
+  --daemon 0 \
+  --check \
+  --nosyslog \
+  --nodetach \
+  -f /etc/fetchmailrc" fetchmail <&- 2>&1