#47 update my revised XSTRUC macros to use %: (aka %00)

[nasm64developer]

With %: (aka %00 -- see SF #809197) becoming an
officially documented NASM feature, my revised
XSTRUC macros should be updated: now XISTRUC or
XIUNION can automatically detect whether they
are preceded by a label (and thus should create
local labels), instead of having to rely on %2.

in XISTRUC:
< __XISTRUC_XIUNION__ XISTRUC,%1,%2
---
> %ifid %:
> %::
> __XISTRUC_XIUNION__ XISTRUC,%1,1
> %else
> __XISTRUC_XIUNION__ XISTRUC,%1,%2
> %endif

in XIUNION:
< __XISTRUC_XIUNION__ XIUNION,%1,%2
---
> %ifid %:
> %::
> __XISTRUC_XIUNION__ XIUNION,%1,1
> %else
> __XISTRUC_XIUNION__ XIUNION,%1,%2
> %endif

Ideally the %2 functionality could be dropped;
however, I decided to keep it in there, because
of existing code, and because I didn't want to
make too many changes to the XSTRUC macros. :)