Make the mechanism for escaping discoverable and customizable

This changes the method of entering an escape sequence:
- raw Ctrl+C gets sent to the VM unimpeded.
- by default, the sequence Ctrl+], Ctrl+C is used to quit the program
  (`^]^C`)
- this can be customized or removed via CLI flags, allowing the string
  be of arbitrary length.
  - i.e. if you `propolis-cli serial -e "beans"` and then type "bea",
    nothing gets sent to the VM after the "b" yet. and then if you type:
    1. "k", the VM gets sent "beak"
    2. '"ns", the VM doesn't get sent anything else, and the client
       exits.
- the client can be configured to pass through an arbitrary prefix
  length of the escape string before it starts suppressing inputs, such
  that you can, for example, mimic ssh's Enter-tilde-dot sequence
  without temporarily suppressing Enter presses not intended to
  start an escape sequence, which would interfere with function:
  `-e '^M~.' --escape-prefix-length=1` (this also works around ANSI
  escape sequences being sent by xterm-like emulators when Enter is
  pressed in a shell that sends a request for such)

Much of this logic, including RawTermiosGuard, has now been factored
out into the https://github.com/oxidecomputer/thouart crate.

Cargo.toml

@@ -113,6 +113,7 @@ softnpu-lib = { git = "https://github.com/oxidecomputer/softnpu", branch = "main
 syn = "1.0"
 tempfile = "3.2"
 thiserror = "1.0"
+thouart = { git = "https://github.com/oxidecomputer/thouart", rev = "656d2a77dd22f7110f513aed2401ebd740b9ef79" }
 tokio = "1"
 tokio-tungstenite = "0.17"
 tokio-util = "0.7"

bin/propolis-cli/Cargo.toml

@@ -15,6 +15,7 @@ propolis-client = { workspace = true, features = ["generated"] }
 slog.workspace = true
 slog-async.workspace = true
 slog-term.workspace = true
+thouart.workspace = true
 tokio = { workspace = true, features = ["full"] }
 tokio-tungstenite.workspace = true
 uuid.workspace = true