MFC 230423, 230612, 232964, 252033, 252036, 252051:

• Replace the beerware license on mdconfig(8) with standard 2-clause BSD.
• Rewrite option parsing in mdconfig(8). This makes it more user-friendly by removing the ordering requirements and adding more descriptive error messages; it also makes it more readable and maintainable.
• Add "-f file" support to listing mode (-l). When a -f option is specified, only md(4) devices which have the specified file as backing store are displayed.
• Use MD_NAME instead of "md".
• Use _PATH_DEV instead of "/dev/".
• Return -1 when the specified backing store file is not found in the md device list.