a change to the config grammar, to specify a device's class (i.e. DV_xxx)
like:
device sd class disk: ...
Also emit an ioconf.h, which contains extern declarations of the
cfdriver structures.
While I'm here, add support for specifying multiple options per defopt
line, as well as specifying the file name in which the options will
appear.
defopt foo bar baz
generates "opt_foo.h", "opt_bar.h", and "opt_baz.h"
defopt opt_mumble.h foo bar baz
generates "opt_mumble.h" which contains all three options.
Also, clean up and fix some bugs in the code that generates header files.