strv: declare iterator of FOREACH_STRING() in the loop

Same idea as 03677889f0.

No functional change intended. The type of the iterator is generally changed to
be 'const char*' instead of 'char*'. Despite the type commonly used, modifying
the string was not allowed.

I adjusted the naming of some short variables for clarity and reduced the scope
of some variable declarations in code that was being touched anyway.

src/nspawn/nspawn-cgroup.c

@@ -22,7 +22,6 @@
 
 static int chown_cgroup_path(const char *path, uid_t uid_shift) {
         _cleanup_close_ int fd = -1;
-        const char *fn;
 
         fd = open(path, O_RDONLY|O_CLOEXEC|O_DIRECTORY);
         if (fd < 0)