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/basic/os-util.c

@@ -170,8 +170,6 @@ int open_extension_release(const char *root, const char *extension, char **ret_p
                         }
                 }
         } else {
-                const char *p;
-
                 FOREACH_STRING(p, "/etc/os-release", "/usr/lib/os-release") {
                         r = chase_symlinks(p, root, CHASE_PREFIX_ROOT,
                                            ret_path ? &q : NULL,