Rearrange daemonization/syslog stuff and make it more resilient.

Rename --background to -D/--daemon, to match other programs (like smbd).

You can now have --syslog even without --daemon.

Avoid using atexit(); try/finally is better.

Don't just close stderr; we'll end up eating error output from ssh!
Instead, redirect stderr to a 'logger' subprocess that will send to syslog.

Delay redirecting stderr until after we know we're daemonizing, so handy
error messages can go to stderr instead of syslog.

Make pidfile stuff more resilient: support already-existing files, files
with strict permissions, outdated files containing an already-dead pid.  Add
a --pidfile option to let you specify the pidfile path.

chdir("/") while daemonizing, so that the filesystem containing $PWD can
later be unmounted without killing the daemon.

fw.done() can't wait on the firewall subprocess on exit when daemonized; we
no longer are the parent of that process.

server.py

@@ -166,6 +166,7 @@ def main():
 
     while mux.ok:
         if hw.pid:
+            assert(hw.pid > 0)
             (rpid, rv) = os.waitpid(hw.pid, os.WNOHANG)
             if rpid:
                 raise Fatal('hostwatch exited unexpectedly: code 0x%04x\n' % rv)