lio_listio(2): send signal even if number of jobs is zero.

Right now, if lio registered zero jobs, syscall frees lio job
structure, cleaning up queued ksi. As result, the realtime signal is
dequeued and never delivered.

Fix it by allowing sendsig() to copy ksi when job count is zero.

PR: 220398
Reported and reviewed by: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27421