Avoid shadowing local variable names.

All of these changes avoid using the same name for two local variables
within a same function.  None of them are actual bugs as far as I can tell,
but any of them could be confusing to the casual reader.

The one in lib/ovsdb-idl.c is particularly brilliant: inner and outer
loops both using (different) variables named 'i'.

Found with GCC -Wshadow.

lib/dynamic-string.c

@@ -147,8 +147,6 @@ ds_put_format_valist(struct ds *ds, const char *format, va_list args_)
     if (needed < available) {
         ds->length += needed;
     } else {
-        size_t available;
-
         ds_reserve(ds, ds->length + needed);
 
         va_copy(args, args_);