libctf: get the encoding of non-ints/fps in the dynamic space right

If you call ctf_type_encoding() on a slice, you are meant to get the
encoding of the slice with the format of the underlying type.  If
you call it on a non-int, non-fp, non-slice, you're meant to get the
error ECTF_INTNOTFP.

None of this was implemented for types in the dynamic space (which, now,
is *all* types in writable containers).  Instead, we were always
returning the encoding as if it were a float, which for all other types
consulted the wrong part of a discriminated union and returned garbage.
(Curiously, existing users were more disturbed by the lack of an error
in the non-int/fp/slice case than they were about getting garbage back.)

libctf/
	* ctf-types.c (ctf_type_encoding): Fix the dynamic case to
	work right for non-int/fps.