Declaration tests for iso646.h, wchar.h, and wctype.h.

We now have a complete test repertoire for what was in C89 (+ the 1995
amendment) and the beginnings of C99 and POSIX.
Better naming convention for POSIX things.

Since the standard I'm actually looking at is the X/Open Single Unix
Standard (which is a superset of POSIX, although the two are much closer
now than they were in 1996), we will now use "x5", "x6", and "x7" for the
1996, 2001, and 2008 revisions, respectively.  These correspond directly
to the appropriate #define _XOPEN_SOURCE value (500, 600, 700).
Fix a handful of errors in the new tests.