src/: Tweak handling of `va_list' pointers in generated functions.

src/: Tweak handling of `va_list' pointers in generated functions.

In method entries, effective method bodies, and delegation trampolines,
the `va_list' argument is named `sod__ap', and a copy is made for
invoking direct methods into a temporary called `sod__tmp_ap'.  The name
`sod__master_ap' is now only used in direct method argument lists and
the `CALL_NEXT_METHOD' machinery.

This reduces the amount of mangling of argument lists which needs to be
done.  It will also make some future changes a little easier.