Fix race condition in qrexec-client-vm handling SIGUSR1

[marmarek]

SIGUSR1 is used to signal qrexec-client-vm to use a single socket (FD 0) for both input and output data (instead of using separate stdin and stdout). But there is a race condition - signal handler is set after spawning child process, so it may try to send the signal before actually handler will be set.

SIGUSR1 handler isn't used yet anywhere, nor even documented, so no need to backport it anywhere. But it is needed for #531