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.

src/basic/fileio.h

@@ -88,3 +88,5 @@ int open_tmpfile_linkable(const char *target, int flags, char **ret_path);
 int link_tmpfile(int fd, const char *path, const char *target);
 
 int read_nul_string(FILE *f, char **ret);
+
+int mkdtemp_malloc(const char *template, char **ret);