Update build dependencies and edition

Use the 2021 Rust edition. To use current version of clap changes were
adapted.

src/posix/create.rs

@@ -1,11 +1,11 @@
 use anyhow::Result;
-use clap::Clap;
+use clap::Parser;
 use log::{info, log_enabled, Level::Info};
 use posixmq::PosixMq;
 use std::fs;
 
 /// Create a POSIX message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Create {
     /// Permissions (octal) to create the queue with
     #[clap(short, long)]

src/posix/info.rs

@@ -1,9 +1,9 @@
 use anyhow::Result;
-use clap::Clap;
+use clap::Parser;
 use posixmq::PosixMq;
 
 /// Print information about an existing message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Info {
     /// Name of the queue
     #[clap(value_name = "QUEUE")]

src/posix/list.rs

@@ -1,11 +1,11 @@
 use anyhow::{anyhow, Result};
 use chrono::{DateTime, Local};
-use clap::Clap;
+use clap::Parser;
 use log::warn;
 use std::{fs, os::unix::fs::PermissionsExt};
 
 /// Print a list of existing message queues
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct List {
     /// Show all parameters
     #[clap(short, long)]

src/posix/recv.rs

@@ -1,13 +1,13 @@
 use anyhow::Result;
 use chrono::{DateTime, Local};
-use clap::Clap;
+use clap::Parser;
 use humantime::Duration;
 use log::info;
 use posixmq::PosixMq;
 use std::str;
 
 /// Receive and print a message from a message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Recv {
     /// Do not block
     #[clap(short, long)]

src/posix/send.rs

@@ -1,11 +1,11 @@
 use anyhow::Result;
 use chrono::DateTime;
-use clap::Clap;
+use clap::Parser;
 use humantime::Duration;
 use log::info;
 
 /// Send a message to a message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Send {
     /// Set a different priority, priority >= 0
     #[clap(short, long, default_value = "0")]

src/posix/unlink.rs

@@ -1,9 +1,9 @@
 use anyhow::Result;
-use clap::Clap;
+use clap::Parser;
 use log::info;
 
 /// Delete a message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Unlink {
     /// Name of the queue
     #[clap(value_name = "QUEUE")]