In keeping with the pattern of mtd@partition, I have added loop@path as a way to specify a loopback device. This way you can do things like mount directories in /system using cramfs from a file otherwise on /system (just one example oof how I'm using it). I specifically went with loop@ rather than adding this feature as a flag as the flags system is designed to set bits in the flags argument to mount: using loop@ fit the model in a much simpler manner and actually feels "correct".
This is a better version of the previously submitted 4045 that also refactors the mtd@ case. The reason for this is that I received comments that I should check for errors and return errors rather that do work in the case of success and fall through, but the mtd@ case wasn't doing that either and it became awkward to design the function so that it was half in one style of error handling and half in another. I also made certain to use inequality comparisons for Unix's -1 error returns rather than checking for -1, refactored my large if statement so as not to have danling parentheses, and disassocited the loop device on mount failure.
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