Provide a custom usage string for --help

The generated output is not very readable. We can do better by providing
a custom string. This makes the binary smaller too.

[Drop indep]

Cargo.toml

@@ -14,7 +14,7 @@ exclude = ["tests/07a-mount-point-excl", "tests/10-example"]
 
 [dependencies]
 anyhow = "1.0.12"
-clap = { version = "4.5", default-features = false, features = ["std", "cargo", "help", "usage", "error-context"] }
+clap = { version = "4.5", default-features = false, features = ["std", "cargo", "help", "error-context"] }
 liboverdrop = "0.1.0"
 rust-ini = ">=0.15, <0.18"
 log = { version = "0.4", features = ["std"] }

src/main.rs

@@ -24,6 +24,11 @@ enum Opts {
 #[rustfmt::skip]
 fn command() -> clap::Command {
     clap::command!()
+        .override_usage("\
+            \tzram-generator --setup-device <device>\n\
+            \tzram-generator --reset-device <device>\n\
+            \tzram-generator dir1 [dir2 dir3]\
+        ")
         .arg(
             clap::arg!(--"setup-device" <device> "Set up a single device")
                 .conflicts_with("reset-device")