The shared with_shutdown_flag helper, and the AtomicBool/Arc/
install_signal_handler imports it uses, only exist when at least one
applet feature (client, server, tui) is enabled. Gate them accordingly
instead of leaving them unconditionally compiled, which broke
`--no-default-features` workspace CI under -D warnings (unused
function/dead-code).
Also correct the README installation section: tui is already part of
the default feature set, so an ordinary `cargo install` already
produces irtt-tui alongside the other three binaries. Document a
lightweight `--no-default-features --features client` install instead
of the previous (incorrect) framing of tui as an opt-in extra, and fix
the same gap in the binaries/features table.
Nils Andreas Svee
d71503e4fd