Fix Unidata#299
The conditions to make this error are the following:
* Two variables with different chunk sizes
* Both variables write on the same unlimited dimension
* The first variable has already written data when the second variable is created

I realize strncat is being eliminated, but in case the elimination isn't done prior to release, this patch should be applied.

Eliminate this warning from clang:
```
TPL/netcdf/netcdf-c/libdispatch/ncuri.c:914:31: warning: for loop has empty body [-Wempty-body]
        for(p=sp,q=sp+1;(*p++=*q++););
                                     ^
TPL/netcdf/netcdf-c/libdispatch/ncuri.c:914:31: note: put the semicolon on a separate line to silence this warning
1 warning generated.
```

Make it clear that NC_MAX_DIMS, NC_MAX_VARS, and NC_MAX_ATTRS limits are not enforced after netcdf-4.5.0 to avoid confusion

…fined types, fixed check of return value