#96 add support for %+0 and %-0

[nasm64developer]

Currently the preprocessor supports %0, as well
as %+n and %-n. Refer to the manual for details.

Now, since %+0 and %-0 never expand to anything
useful, they can be assigned to another meaning.

For example, they could be used to refer to the
number of completed or remaining iterations of a
preprocessor loop. (Granted, the user can track
this information on his/her own by maintaining a
set of two preprocessor variables for a loop --
but a NASM-provided means seems much nicer. :-)

Attached find the necessary changes (note: these
require SF RFE #872331), as well as testcases.

[nasm64developer]

changes

[nasm64developer]

testcases

[nasm64developer]

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See SF RFE #1365092 for newer code.