Self-Test: Run nginx

[nik9000]

Adds a self-test that runs nginx and doesn't basic sanity checks on the
files it serves. This is nice to make sure that --open works but
also it should serve as another line of defence against making invalid
nginx config files. This is important as those nginx config files will
be used on the web site and we'd really prefer not to bring that down.

To get this to work consistently I have to drop the "don't run twice"
behavior of build_docs.pl. That behavior is implemented by checking
for a pidfile on startup. If the file is there it checks if ps for
that pid comes back with a process that looks like the running one. When
the perl process terminates naturally it destroys the file. This is
known to cause trouble when a server loses power and can't delete the
file and the process is started on startup. Startup is fairly
predictable and it is common that the process would end up with the same
pid so the ps check will think that the new process collides with
itself.

Pulling a server's power from the wall is fairly similar to what
happens when the docker process that runs the --self-test comes to the
end and destroys that docker image. This hasn't come up before because
build_docs.pl always terminates fairly quickly. It doesn't when you do
--open. I tried to get something that consistently removed the pidfile
but such a thing is complex in make, I think. So I just removed it.

"Don't run twice" wasn't really buying us much anyway. At this point we
run build_docs on immutable infrastructure in CI and Jenkins manages
whether or not to run the build concurrently. We have much more precise
control there which is what we want anyway. It might have some value
during --self-test to make sure that we don't leave build_docs.pl
running between tests. At this point we shoot it if the test is
successful and let it die if the test fails. Which isn't great but it
should work.

[ghost]

PID files are lock files, and lock files are trouble. Good riddance.

I usually try to do some tricky integration interrogation of the process list to avoid PID files, and thus stale locks. The output of ps -eo comm and ps -eo command are favorites of mine.

[nik9000]

Thanks for reviewing @Jarpy!