Remove dead code
startProcess: Make writing error messages from the child more robust
nix-daemon: Use a thread instead of SIGPOLL to catch client disconnects

The thread calls poll() to wait until a HUP (or other error event)
happens on the client connection. If so, it sends SIGINT to the main
thread, which is then cleaned up normally. This is much nicer than
messing around with SIGPOLL.
nix-store --serve: Monitor for client disconnects

This is necessary because build-remote.pl now builds via ‘nix-store
--serve’. So if a build hangs without writing to stdout/stderr, and
the client disconnects, then we need to detect that.
nix-daemon: Simplify stderr handling
nix-daemon: Less verbosity
Pass -pthread only for programs that need it