[CM] Identify binary plugins

[bil at ccrma.Stanford.EDU]

Thanks again, but I am not comfortable with this.
It means building in magic numbers and assuming
that everyone uses them, but I'd like s7 to work
anywhere there's a C compiler and (in this context)
dlopen, and be able to handle changes in the future
without maintaining an ever-growing list of magic
numbers.  (And it means any file starting with "MZ"
will be treated as an object file).