strv: declare iterator of FOREACH_STRING() in the loop

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.