path-util: add path_extract_directory(), to match path_extract_filename()

These two together are a lot like dirname() + basename() but have the
benefit that they return clear errors when one passes a special case
path to them where the extraction doesn't make sense, i.e. "", "/",
"foo", "foo/" and so on.

Sooner or later we should probably port all our uses of
dirname()/basename() over to this, to catch these special cases more
safely.

src/basic/path-util.h

@@ -147,6 +147,7 @@ int fsck_exists(const char *fstype);
 char* dirname_malloc(const char *path);
 const char *last_path_component(const char *path);
 int path_extract_filename(const char *p, char **ret);
+int path_extract_directory(const char *p, char **ret);
 
 bool filename_is_valid(const char *p) _pure_;
 bool path_is_valid(const char *p) _pure_;