Fix GH-10992: Improper long path support for relative paths

Relative paths are passed to the ioutils APIs, these are not properly
converted to long paths. If the path length already exc

Fix GH-10992: Improper long path support for relative paths

Relative paths are passed to the ioutils APIs, these are not properly
converted to long paths. If the path length already exceeds a given
threshold (usually 259 characters, but only 247 for `mkdir()`), the
long path prefix is prepended, resulting in an invalid path, since long
paths have to be absolute. If the path length does not exceed that
threshold, no conversion to a long path is done, although that may be
necessary.

Thus we take the path length of the current working directory into
account when checking the threshold, and prepend it to the filename if
necessary.

Since this is only relevant for NTS builds, and using the current
working directory of the process would be erroneous for ZTS builds, we
skip the new code for ZTS builds.

Closes GH-16687.

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