syslogd: Do not reap child processes

Use the SA_NOCLDWAIT sigaction() flag to disable the creation of
zombie processes. This leaves no reason to wait() on children, so
remove the reaping routine.

Previously, deadq entries were removed in reapchild(). Move deadq
removal into markit() after SIGKILL is sent.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41415