bus: Add __BUS_ACCESSOR_DEFAULT

[jhb]

Summary

This macro is similar to __BUS_ACCESSOR in that it creates three
helper routines for an ivar, but the "get" wrapper returns a default
value if BUS_READ_IVAR does not return a value.

[imp]

sys/sys/bus.h

1030

no problem with C having 'default' as a keyword? I know it's cpp, just read a little odd.

[jhb]

sys/sys/bus.h

1030

I think it doesn't make it into what the compiler sees as it is only for cpp.