server: Cleanly shut down senders on exit.

server: Cleanly shut down senders on exit.

This introduces ->shutdown for struct sender. Each of the three senders
implements the new method to close file descriptors and to deallocate
the resources occupied by the sender. The method is only called on
exit via the new vss_shutdown() from both server and command handler
context after schedule() returns.

We need to introduce another helper, process_is_command_handler(), to
distinguish between the two callers.