Changes:
* replace `iotuil` elements with the ones from `os` package;
* replace `os.Filemode` with `fs.FileMode`;
* use `signal.NotifyContext` instead of `NewGracefulContext` (removed).
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Flag `-i` is deprecated in go1.16 and it
has pretty much no effect since go1.10 when
build cache was introduced. Read more about
this at https://golang.org/doc/go1.16
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
