With SOCK_DGRAM sockets, the helper doesn't stop as there is no error
received when the socket is closed on the daemon side.
Add a QUIT operation to the protocol which is requested when the daemon
is exiting. It has no response. Register the stopping function with
atexit() to stop the helper even when the daemon is not exiting cleanly,
e.g. due to a fatal error.
Split out the sending part of the function into send_request() and
rename it to submit_request(). This will be useful to send a request
without waiting for a response.
Also, remove the fd parameter from the functions and just use helper_fd
directly.
SOCK_SEQPACKET is preferred over SOCK_DGRAM for communication with the
helper as the process will get an error when the other end of the socket
is closed. It's not supported on all platforms.
If SOCK_SEQPACKET is defined, try creating the pair of sockets with this
type first and if that fails, fall back to SOCK_DGRAM.
