sbitmap cleanups

[Richard Kenner]

    Yes but nothing says that you aren't coming GCC with some other
    compiler, that might intepret _X as a command to delete the source and
    launch the missles.  Please change this back to the original X_ or a
    similar convention that does not use reserved names.  Just because it
    happens to work for your example on the compilers you've tested it
    against, doesn't make it legal.

OK.  I went back and checked and found that you're right that this
seems to be the more common convention: for some reason, I thought the
other way around was more common.