bug#22881: 25.0.50; Add more granularity to `save-silently'

[Oleh Krehel <oleh@oremacs.com>]

I'd like to configure `save-silently' to customize the message of
`save-buffer'.

Here's the part I don't want to have:

    Saving file /usr/local/foo...

Here's the part I want to keep:

    Wrote /usr/local/foo

Both messages are emitted in `basic-save-buffer-2', which checks
`save-silently' twice whether to emit each message. But so far, it's an
all-or-nothing deal.

I would suggest modifying `save-silently' to be a verbosity level:

- 0 or nil means nothing gets printed.
- 1 means "Wrote ..." is printed.
- 2 means both messages are printed.

The main reason why I want to skip the "Saving file..." is that both
messages are printed in quick succession, and what's worse, sometimes
the first message takes more than one line because of the long file
name. And then I get a 0.1s window re-size by one line flicker.