It provides path to a directory suitable for storing temporary files
which satisfies the following conditions:
- The directory exists.
- The return path has trailing directory separator.
It is based on the code used in the appdir.c to get the temporary
directory.
For the C++ people: this is similar to the temp_directory_path()
from the std::filesystem.
No functional changes expected as it is a new code which is planned
to be used on other places as a followup development.
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