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-mount.c

@@ -404,7 +404,7 @@ int tmpfs_patch_options(
 }
 
 int mount_sysfs(const char *dest, MountSettingsMask mount_settings) {
-        const char *full, *top, *x;
+        const char *full, *top;
         int r;
         unsigned long extra_flags = 0;
 
@@ -464,7 +464,7 @@ int mount_sysfs(const char *dest, MountSettingsMask mount_settings) {
         /* Create mountpoint for cgroups. Otherwise we are not allowed since we
          * remount /sys read-only.
          */
-        x = prefix_roota(top, "/fs/cgroup");
+        const char *x = prefix_roota(top, "/fs/cgroup");
         (void) mkdir_p(x, 0755);
 
         return mount_nofollow_verbose(LOG_ERR, NULL, top, NULL,