to fix hostname validation in IO::Socket::SSL if IPs not included in certificates

(most likely fails as we use IPs as hostnames)

accept if running with connect-test-only

After executing the HOST-DOWN command we have to wait for the director
OK response, before we can issue the HOST-FLUSH, otherwise the second
command may not be processed.

Avoids output delays when poolmon runs in as a systemd service.

IO::Socket::IP duplicates the functionality of INET6 but is less likely to be packaged by distros.

Use doveadm director up/down commands to change the state.

These are available in Dovecot v2.2.19 and newer. Using these makes it
possible for admins to manually change the hosts' weights without poolmon
overwriting them.

Removed handling of __DIE__ signal.

I removed handling of the __DIE__ signal because it
was causing problems. Because the signalhandler for
the __DIE__ signal was calling remove_pidfile() the
forks ended up removing the PID-file whenever a
backend timed out. Therefore I added a check to
remove_pidfile() to ensure that only the parent
process is allowed to actually remove the PID-file.

I also addded an END statement to take care of
PID-file cleanup.

Conflicts:
	poolmon

added Ubuntu upstart script

 - Remove dependency on external IMAP module
 - Add support for POP3 login check

New checks are just very basic and only support simple username/password
authentication, but I was not a huge fan of requiring full POP3/IMAP
modules just to get a banner and send credentials.

This cuts down on a significant amount of duplicate code, but does force us
to require Net::IMAP::Simple, as we are now doing more than just a simple connect
or SSL handshake.

- Includes login checks from dfwarden@gmail.com - thanks David!