syslogd: Watch piped procdescs in kqueue

[jfree]

Summary

When a piped command is launched, register its process descriptor with
the kqueue.

The main kevent() loop is already setup to reap processes that trigger
the EVFILT_PROCDESC filter

[markj]

Just a generate comment: when casper starts handling the popen(), syslogd will no longer be able to use waitpid() to reap the process, since it won't be the parent process.