Werk #15417: mk_logwatch: Correctly handle UNIX paths with non-UTF-8 characters

On UNIX, filenames may consist of arbitrary bytes.

To display filenames in Checkmk, the mk_logwatch plugin assumes a UTF-8 encoding and previously replaced each non-UTF-8 (hence non-printable) byte with the replacement character �.
This lead to the problem that multiple filenames were represented by the same name in Checkmk.
As a result, the logfiles got displayed in one service, with the logfile contents merged to one log view.

In order to handle this situation, mk_logwatch now uses a backslash escape sequence instead of the replacement character.
Non-UTF-8 bytes are now represented by their hexadecimal value with a \x prefix.
E.g., monitoring the two files (as shown with ls, i.e., with octal escape sequence) 'my'$'\300\201''file' and 'my'$'\300\200''file now yields two services Log my\xc0\x80file and Log my\xc0\x81file, while it used to yield only one service Log my��file before.

Due to the new service name, you have to discover the newly named services if you are affected by this change.

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