syslogd: Terminate pipe processes gracefully

[jfree]

Summary

Pipe actions spawn processes based on the command provided in the
syslogd configuration file. When a HUP signal is received, enter
these processes into the deadq instead of immediately killing them.
This matches the behavior of syslogd prior to it being Capsicumized.

Fixes: d2d180fb7736

Test Plan

Using test provided by bug report:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291557

$ logger -p local2.warning -t logger message1
$ killall -1 syslogd
$ logger -p local2.warning -t logger message2
$ killall -1 syslogd

$ tail /var/log/messages
Dec 22 06:44:54 CURRENT logger[6027]: message1
Dec 22 06:44:55 CURRENT pipe-test[6032]: 6028 START
Dec 22 06:44:55 CURRENT pipe-test[6036]: 6028 Dec 22 06:44:54 CURRENT logger[6027]: message1
Dec 22 06:44:56 CURRENT pipe-test[6041]: 6028 END
Dec 22 06:44:59 CURRENT logger[6045]: message2
Dec 22 06:44:59 CURRENT pipe-test[6050]: 6046 START
Dec 22 06:44:59 CURRENT pipe-test[6054]: 6046 Dec 22 06:44:59 CURRENT logger[6045]: message2
Dec 22 06:45:02 CURRENT pipe-test[6061]: 6046 END

[markj]

Is it possible to convert the test scenario into a regression test case?

[jfree]

Tests added. I forgot to add the review link into the commit message... oops. Closing this.