Fix possible deadlock on closing the watcher on kqueue (#230)

Fix possible deadlock on closing the watcher on kqueue (#230)

* avoid deadlocks on Close()

raw channel work (not inside a select) should
always be prohibited in production code, as
it readily causes deadlocks on shutdown.

Also adds the test TestWatcherClose from #145.
This request duplicates that test, with two
lines fixed to address the houndcli-bot review
concerns.

Fixes #187
Fixes #145

* cleanup and simpler test

* also fix #225

* fix tests