Support slice numbers in device names. The syntax is `<driver name>
[<unit number>] ['s' <slice number>] [<partition letter>]'. Only
`['s' <slice number>]' is new here. The slice number defaults to 0
so that there is no change in the output from config if this new
feature is not used.
Replace some magic disk numbers by `dk' slice and label macros.
mkswapconf.c:
Improve the output formatting:
Generate <> style includes.
Print minor numbers in hex so that slice numbers are easy to see and edit.
Print the rootdev and dumpdev names in comments like the swapdev names.
