dd(1): Sync with FreeBSD

Some highlights:
* Add the "fillchar" operand.
* Add the "speed" operand to limit the speed.
* Support to specify the output data parity in the "conv" operand.
* Add tests,

dd(1): Sync with FreeBSD

Some highlights:
* Add the "fillchar" operand.
* Add the "speed" operand to limit the speed.
* Support to specify the output data parity in the "conv" operand.
* Add tests, which can be run with "make test".

I adjusted the device names in the examples of the man page. Some minor
tweaks (e.g., styles, function prototypes, comments) were also applied.

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