Werk #1012: Fix quoting of backslashes in custom checks with nagios core

When you are using Nagios as montitoring core then backslashes contained in custom_checks (Classical active and passive Nagios checks) would be interpreted by Nagios. That way single backslashes would usually vanish und you would have to duplicate them.

The Check_MK Micro Core never interpreted these backslashes so it was transparent for the user.

This fix changes the behaviour of Check_MK in a way that also with the Nagios core you need only one backslash. Backslashes are transparent now.

NOTE: If you have custom checks that contain backslashes (for example as part of names or passwords) then you need to remove duplicate backslashes after this update!

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