Move (qrexec-agent version of) do_exec to qrexec-agent.c, move
handle_handshake to qrexec-agent-data.c (common to all agent binaries).
Fix indentation (tabs -> spaces).
The main advantage is possible use of single socket for both stdin and
stdout. This is strictly required for using USBIP over qrexec.
For compatibility qrexec still creates three socket pairs (instead of
pipes) for stdin/out/err respectively. When qrexec-agent receives
SIGUSR1, it will close stdout socket and use stdin socket for both
directions.
Some additional work is needed here to actually allow child process to
send that signal - qrexec is running as root, but child as "user" in
most cases.
