When message received on `diagnosticSpiderKill` I was failing to set
`diagnosticSpiderDummy` false again, so the next attempt to start a
dummy would return early, so there would be nothing listening on the
`diagnosticSpiderKill` channel after the first dummy is killed.

With a buffered channel, the second kill request was ignored, but stuck
in the channel, and so a third kill request would block; that would
block the invoker's channel.

This change makes some channels synchronous, as it's easier for
debugging and making problems reveal themselves, even if synchronous
might not be strictly needed.  Being synchronous does make it easier to
reason about the dummy-spider vs spider switch-out.  All that, just to
make sure that a diagnostics request would always have something
draining the channel.

Also: made the persisted lock a `sync.RWMutex` instead of a `sync.Mutex`
and use read-locks as appropriate: in theory, permits high web query
rates, since each web request locks this mutex to get the persisted
state.

Scott points out that these binaries are more widely applicable than
just for Debian.

[ci skip]

Scott Grayban is providing pre-built binaries for Debian.

The code contributions section was too aggressive in the phrasing used
for code submission authority; I knew that at the time, but went with
it.  Rephrased to remove the negative terms and confrontational attitude
and instead says what the submitter is implicitly doing by making a
submission.

[ci skip]

I read:
http://www.infoworld.com/d/open-source-software/github-needs-take-open-source-seriously-208046
and got concerned about exposure.  Bleh.

[ci skip]

This is what I use for DNS zonefile generation.

[ci skip]

Decided to write test for this as simple to do.  Good thing I did.
It did not work.
Never assigned the built list to the global.  *sigh*
Fixed.

Git config of extra build targets. Signal safe pop.

hooks.PrecommitGoBuildExtraTarget to add 1 or more extra build targets;
use this if you have a binary target and "go build" doesn't see it
(because of the "// +build ignore" needed to make the build system happy
with the multiple packages).

Also, handle SIGINT, SIGTERM and pop the stash cleanly then too.

Made slightly more awkward by more recent zsh deciding that trap of EXIT
pseudo-signal inside a function should refer to return from the
function, not the global EXIT hook.  *sigh*

[ci skip]

Asked for opinions, Derek spoke up, his response matched what I was
thinking, so go with that.  "vet" and "fmt" are run by default, "build"
and "test" have to be requested.

[ci skip]

[ci skip]

Warn, but don't fail.

[ci skip]

We want to invoke pre-commit on the code that's actually being
submitted, so we actually test what will be part of the commit.

This does that, unless PRE_COMMIT_DIRTY=true is in environ.

[ci skip]

pre-commit uses function, now it does "go vet", "go fmt"..., "go build",
"go test".  Slow but safer.  All have knobs to disable.

Add second test!  depth_test.go
 * Confirmed works whether "go test" invoked in dir, in
   ${GOPATH%%:*}/src or anywhere else: if not in-dir, the package given
   is searched for in $GOPATH and tests are invoked in-dir, so relative
   filenames pointing to data/ are fine.

Put copyright notice into countries_test.go