environment-generator: new generator to peruse environment.d

Why the strange name: the prefix is necessary to follow our own advice that
environment generators should have numerical prefixes. I also put -d- in the
name because otherwise the name was very easy to mistake with
systemd.environment-generator. This additional letter clarifies that this
on special generator that supports environment.d files.

src/shared/path-lookup.c

@@ -503,8 +503,7 @@ int lookup_paths_init(
                         append = true;
                 }
 
-                /* FIXME: empty components in other places should be
-                 * rejected. */
+                /* FIXME: empty components in other places should be rejected. */
 
                 r = path_split_and_make_absolute(e, &paths);
                 if (r < 0)