syslogd: Use process descriptors

[jfree]

Summary

Prepare for program Capsicumization by storing process descriptors
instead of pids. Signal delivery is not permitted in capability mode,
so we can use pdkill(2) to terminate child processes.

[jfree]

Process descriptors for piped processes are registered with kevent() in this patch:

https://reviews.freebsd.org/D41389

[jfree]

Update after rebase

[markj]

usr.sbin/syslogd/syslogd.c

1958–1959

deadq_enter() saves the process descriptor in the entry, but the subsequent close_filed() call will close the process descriptor, so we're left with a dangling fd.

[jfree]

Move procdesc closure into deadq_remove() so we avoid dangling, invalid procdescs

[markj]

deadq_enter() and close_filed() are now always called together. Should the latter call the former directly? If not, I'd suggest adding a comment to close_filed() explaining that the procdesc is closed elsewhere, otherwise it looks like it's being leaked.

[jfree]

Move deadq_enter() into close_filed()