xtra: Avoid undefined ## usage.

xtra: Avoid undefined ## usage.

According to the C spec, every use of the ## operator has to produce a
valid preprocessing token, otherwise the behaviour is undefined.  It is
not enough to merely ensure that a sequence of ## operators results in
a valid token at the end.

This matters in practice, as at least some versions of the Sun Studio
compiler will turn:

  #define PASTE(a, b) a ## b
  #define PASTE2(a, b) PASTE(a, b)

  PASTE2(uint_least, PASTE(8, _t))

into two tokens:

  uint_least8 _t

instead of uint_least8_t as desired.  The results are not good for
the XTRA_PACKED_LOPTS macro.  Fortunately, it is straightforward to
rearrange the expansions to avoid this problem.