strv: declare iterator of FOREACH_STRING() in the loop
    
    Same idea as 03677889f0ef42cdc534bf3b31265a054b20a354.
    
    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.