wrong usage message for mlprof

[Matthew Fluet]

> The  usage  message in mlprof isn't right.  It indicates no space between the
> `-d' and the number, but a space is required.  (Note, I like the space  being
> required, but the usage message needs fixing.)

Done.

> By  the  way,  I  changed  my  copy of src/runtime/prof.c so that the file it
> generates is called `mlmon.out' instead of `mlprof.out'.  Hopefully the  name
> will  be  less confusing.  Also the usage message in the mlprof program seems
> to think that the file is a gmon.out file, which it  isn't.   (There  was  an
> error in prof.c such that it used glmon.out in some error message).

I made the same changes to my copy of prof.c.  Should we change the magic
value to "MLton mon\n\n"?

I'm looking into the static label problem, although I'm not exactly sure
what the issue is.  The problem with measuring time in code not generated
by the backend is that I'm just guessing where one function begins and
ends.  What should the difference between executing with and without -s
be?