Finish implementing -d/--device/--device-path flag to print variable
as if it were a device path.

Remove language about a=b syntax on the command line. This will not be
implemented due to its limited usefulness. UEFI variables are binary
blobs, on the whole, and a simple work around exists for
strings. Clarify that the new value of the variable is taken from
stdin. Update manual with history.

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