Avoid using reserved identifiers.

In C, all identifiers beginning with an underscore followed by either
another underscore or an uppercase letter are reserved for use by the
implementation in all contexts.

Let's avoid such identifiers in the program proper.  Since most problems
are declarations like 'typedef struct _Foo Foo;' we can just change this
without too much issue.

The generated code still does this sort of thing, but unfortunately
would be a more significant change as this behaviour is documented.