daemon: use kqueue for all events

Refactor daemon to use kqueue/kevent instead of signals.

This changes allows to simplify the code in several ways:

• the execution flow is now linear, no async events.
• several variables became redundant and got removed.
• all event handling is now concentrated inside of the event loop, which makes code reading and comprehension easier.
• new kqueuex(2) call is used for CLOEXEC, but maintained closing the kq fd prior to execve() to ease later MFC

No UX/API changes are intended.

Reviewed by: kevans
Pull Request: https://github.com/freebsd/freebsd-src/pull/701