mdconfig(8) argument parsing.

[Edward Tomasz Napierała]

Patch below changes the way mdconfig(8) parses its arguments:
it removes the ordering requirement and makes error messages
more descriptive; it also makes the code more readable by
getting rid of the 'cmdline' variable.

Now, the mdconfig(8) syntax is somewhat weird, and I'm not sure
I tested all the ways people use it.  Thus, testing is welcome.

http://people.freebsd.org/~trasz/mdconfig-parsing.diff

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