libzfs: sendrecv: send_progress_thread: handle SIGINFO/SIGUSR1

POSIX timers target the process, not the thread (as does SIGINFO),
so we need to block it in the main thread which will die if interrupted.

Ref: https://101010.pl/@ed1conf@bsd.network/110731819189629373
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #15113