scripts: refactored scripts located under target/bin/ (#2500)

* refactored scripts located under `target/bin/`

The scripts under `target/bin/` now use the new log and I replaced some
`""` with `''` on the way. The functionality stays the same, this mostly
style and log.

* corrected fail2ban (script and tests)

* corrected OpenDKIM log output in tests

* reverted (some) changes to `sedfile`

Moreover, a few messages for BATS were streamlined and a regression in
the linting script reverted.

* apple PR feedback

* improve log output from `fail2ban` script

The new output has a single, clear message with the '[  ERROR  ]  '
prefix, and then output that explains the error afterwards. This is
coherent with the logging style which should be used while providing
more information than just a single line about IPTables not functioning.

* simplified `setquota` script

* consistently named the `__usage` function

Before, scripts located under `target/bin/` were using `usage` or
`__usage`. Now, they're using `__usage` as they should.

* improved `sedfile`

With `sedfile`, we cannot use the helper functions in a nice way because
it is used early in the Dockerfile at a stage where the helper scripts
are not yet copied. The script has been adjusted to be canonical with
all the other scripts under `target/bin/`.

* fixed tests

* removed `__usage` from places where it does not belong

`__usage` is to be used on wrong user input, not on other failures as
well. This was fixed in `delquota` and `setquota`.

* apply PR review feedback

target/bin/fetchmailrc_split

@@ -14,7 +14,7 @@ DEFAULT_FILE="${FETCHMAILRCD}/defaults"
 
 if [[ ! -r "${FETCHMAILRC}" ]]
 then
-  echo "Error: File ${FETCHMAILRC} not found"
+  _log 'error' "File '${FETCHMAILRC}' not found"
   exit 1
 fi
 
@@ -22,7 +22,7 @@ if [[ ! -d ${FETCHMAILRCD} ]]
 then
   if ! mkdir "${FETCHMAILRCD}"
   then
-    echo "Error: Unable to create folder ${FETCHMAILRCD}"
+    _log 'error' "Unable to create folder '${FETCHMAILRCD}'"
     exit 1
   fi
 fi