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/fs-util.h

@@ -84,3 +84,10 @@ enum {
 };
 
 int chase_symlinks(const char *path_with_prefix, const char *root, unsigned flags, char **ret);
+
+/* Useful for usage with _cleanup_(), removes a directory and frees the pointer */
+static inline void rmdir_and_free(char *p) {
+        (void) rmdir(p);
+        free(p);
+}
+DEFINE_TRIVIAL_CLEANUP_FUNC(char*, rmdir_and_free);