Fix incompatibilities when upgrading to clap 4.0

See the clap CHANGELOG:
https://github.com/clap-rs/clap/blob/v4.0.18/CHANGELOG.md.

`parse(from_os_str)` for a PathBuf just becomes `value_parser`:

  - For `#[clap(parse(from_os_str)]` for `PathBuf`, replace it with
    `#[clap(value_parser)]` (as mentioned earlier this will call
    `value_parser!(PathBuf)` which will auto-select the right `ValueParser`
    automatically).

For `min_values = 2`, this becomes `num_args(2..)`. This is a new
interface that replaces a number of old ones:

    **`Arg::num_args(range)`**

    Clap has had several ways for controlling how many values will be
    captured without always being clear on how they interacted,
    including
    - `Arg::multiple_values(true)`
    - `Arg::number_of_values(4)`
    - `Arg::min_values(2)`
    - `Arg::max_values(20)`
    - `Arg::takes_value(true)`

    These have now all been collapsed into `Arg::num_args` which accepts
    both single values and ranges of values.  `num_args` controls how
    many raw arguments on the command line will be captured as values
    per occurrence and independent of value delimiters.

    See [Issue 2688](clap-rs/clap#2688) for
    more background.

`validator` has been removed, see
https://epage.github.io/blog/2022/06/clap-32-last-call-before-40/. We
were previously using `validator = ...` to set a function to validate
that the value of the DSCP parameter was in the allowed range (0-63).
This no longer requires an entire function and we can just write
`value_parser = clap::value_parser!(u8).range(0..63)`. However, we lose
the ability to detect Windows usage (where we don't support setting the
DSCP value) at argument parsing time.

For setting the verbosity level based on the number of `-v`'s:

  - For `#[clap(parse(from_occurrences))]` replaced with `#[clap(action
    = ArgAction::Count)]` though the field's type must be `u8` (#3794)

src/bin/dnstap-dump/main.rs

@@ -1,4 +1,4 @@
-// Copyright 2021 Fastly, Inc.
+// Copyright 2021-2022 Fastly, Inc.
 
 use anyhow::{bail, Result};
 use chrono::NaiveDateTime;
@@ -26,7 +26,7 @@ struct Opts {
     #[clap(short = 'r',
            long = "read",
            name = "FILE",
-           parse(from_os_str),
+           value_parser,
            value_hint = ValueHint::FilePath)
     ]
     file: PathBuf,

src/bin/dnstap-replay/main.rs

@@ -2,7 +2,7 @@
 
 use anyhow::Result;
 use async_channel::{bounded, Receiver, Sender};
-use clap::{Parser, ValueHint};
+use clap::{ArgAction, Parser, ValueHint};
 use log::*;
 use std::net::SocketAddr;
 use std::path::PathBuf;
@@ -88,7 +88,9 @@ pub struct Opts {
     dns: SocketAddr,
 
     /// DSCP value to set on outgoing queries
-    #[clap(long, name = "DSCP code point", validator = is_dscp)]
+    #[clap(long,
+           name = "DSCP code point",
+           value_parser = clap::value_parser!(u8).range(0..63))]
     dscp: Option<u8>,
 
     /// HTTP server socket to listen on for stats and reporting
@@ -115,8 +117,8 @@ pub struct Opts {
     #[clap(long = "match-status-files",
            name = "STATUS-FILE",
            required = false,
-           min_values = 2,
-           parse(from_os_str),
+           num_args(2..),
+           value_parser,
            value_hint = ValueHint::FilePath)
     ]
     status_files: Vec<PathBuf>,
@@ -126,23 +128,8 @@ pub struct Opts {
     unix: String,
 
     /// Increase verbosity level
-    #[clap(short, long, parse(from_occurrences))]
-    verbose: usize,
-}
-
-#[cfg(unix)]
-fn is_dscp(val: &str) -> Result<(), String> {
-    // Parse 'val' as an integer, but only allow values between 0 and 63 inclusive since the DSCP
-    // field is a 6-bit quantity.
-    match val.parse() {
-        Ok(0..=63) => Ok(()),
-        _ => Err(String::from("DSCP code point must be in the range [0..63]")),
-    }
-}
-
-#[cfg(not(unix))]
-fn is_dscp(_val: &str) -> Result<(), String> {
-    Err(String::from("Cannot set DSCP values on this platform"))
+    #[clap(short, long, action = ArgAction::Count)]
+    verbose: u8,
 }
 
 impl Server {
@@ -259,7 +246,7 @@ fn main() -> Result<()> {
     let opts = Opts::parse();
 
     stderrlog::new()
-        .verbosity(opts.verbose)
+        .verbosity(opts.verbose as usize)
         .module(module_path!())
         .init()
         .unwrap();