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/shared/mount-setup.c

@@ -126,9 +126,6 @@ bool mount_point_is_api(const char *path) {
 }
 
 bool mount_point_ignore(const char *path) {
-
-        const char *i;
-
         /* These are API file systems that might be mounted by other software, we just list them here so that
          * we know that we should ignore them. */
         FOREACH_STRING(i,
@@ -515,7 +512,6 @@ int mount_setup(bool loaded_policy, bool leave_propagation) {
          * use the same label for all their files. */
         if (loaded_policy) {
                 usec_t before_relabel, after_relabel;
-                const char *i;
                 int n_extra;
 
                 before_relabel = now(CLOCK_MONOTONIC);