This option, when added via
  TESTS_ENVIRONMENT = [...] $(top_builddir)/run --test
allows us to run the tests and only print the full output (including
debugging etc) when the test fails.

The start_test_thread function never returns, it is always
cancelled by the main thread.

The presumption is that all file descriptors should be created with
the close-on-exec flag set.  The only exception are file descriptors
that we want passed through to exec'd subprocesses (mainly pipes and
stdin/stdout/stderr).

For open calls, we pass O_CLOEXEC as an extra flag, eg:

  fd = open ("foo", O_RDONLY|O_CLOEXEC);

This is a Linux-ism, but using a macro we can easily make it portable.

For sockets, similarly:

  sock = socket (..., SOCK_STREAM|SOCK_CLOEXEC, ...);

For accepted sockets, we use the Linux accept4 system call which
allows flags to be supplied, but we use the Gnulib 'accept4' module to
make this portable.

For dup, dup2, we use the Linux dup3 system call, and the Gnulib
modules 'dup3' and 'cloexec'.

Update all copyright dates to 2012.

Change the test for duplicate drives so that you're allowed to
add /dev/null multiple times.  This corresponds to traditional
usage.

This amends commit be47b66c3033105a2b880dbc10bfc2b163b7eafe.