util-lib: add easy helpers for temporary directories that rmdir()ed via _cleanup_

util-lib: add easy helpers for temporary directories that rmdir()ed via _cleanup_

This adds mkdtemp_malloc() that is a combination of mkdtemp() plus strdup(). It
initializes its return paremeter only if the temporary directory could be
created successfully, so that the parameter is exactly non-NULL when the
directory exists.

rmdir_and_free() and rmdir_and_freep() are also added, and the latter may be
used inside of _cleanup_ for such a directory string variable, to automatically
rmdir() the directory if it is non-NULL when the scope exits.

rmdir_and_free() is similar to the existing rm_rf_and_free() however, is only
removes a single directory and does not operate recursively.