Clean up Unix Pageant's setup and teardown.

Clean up Unix Pageant's setup and teardown.

I've moved the listening socket setup back to before the lifetime
preparations, so in particular we find out that we couldn't bind to
the socket _before_ we fork. The only part that really needed to come
after lifetime setup was the logging setup, so that's now a separate
function called later.

Also, the random exit(0)s in silly places like x11_closing have turned
into setting a time_to_die flag, so that all clean exits funnel back
to the end of main() which at least tries to tidy up a bit afterwards.

(Finally, fixed a small bug in testing the return value of waitpid(),
which only showed up once we didn't exit(0) after the first wait.
Ahem.)