timeout(1): only start the child command after the parent is fully set up

Since the default disposition for SIGCHLD is ignore, the prematurely
exited child would cause SIGCHLD dropped. This makes timeout(1) hang,
because REAP_STATUS reports a zombie not waited for, but SIGCHLD for it
was already lost, so the main loop cannot exit, instead calling into
sigsuspend().

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D50752