tree-wide: make FOREACH_DIRENT_ALL define the iterator variable

The variable is not useful outside of the loop (it'll always be null
after the loop is finished), so we can declare it inline in the loop.
This saves one variable declaration and reduces the chances that somebody
tries to use the variable outside of the loop.

For consistency, 'de' is used everywhere for the var name.

src/basic/cgroup-util.c

@@ -199,8 +199,6 @@ int cg_enumerate_subgroups(const char *controller, const char *path, DIR **_d) {
 }
 
 int cg_read_subgroup(DIR *d, char **fn) {
-        struct dirent *de;
-
         assert(d);
         assert(fn);