Updated net/rabbitmq to 2.6.1.

Various pkgsrc fixes:

- Fix mangled PLIST from the previous commit.
- Depend on coreutils for readlink, which is used in rabbitmq-env.
- Depend on bash, which is assumed throughout the scripts.
- Make sure the shell is passed properly to make/install targets.
- Fix Python usage (add Python 2.7) and clean up other bits.

RabbitMQ changelog:

2.6.1 bug fixes
- The broker failed to (re)start on reboot on systems that keep
  /var/run on a temporary file systems, e.g. Ubuntu.
- The Windows service failed to increase the Erlang process limit,
  limiting the broker to a few thousand queues, connections and
  channels.

2.6.0 bug fixes
- Upgrading from RabbitMQ 2.1.1 to any later release could break if
  there were durable queues with persistent messages present.
- On very slow machines, starting rabbit via the supplied init scripts
  could fail with a timeout.
- Rabbit could fail to stop (when asked to do so) in the presence of
  some plug-ins (e.g. shovel).
- 'ram' nodes in a cluster could consume ever increasing amounts of
  disk space.
- The presence of fast consumers on a queue could significantly delay
  the addition of new consumers.
- When a client was issuing a tx.commit in one channel, and
  simultaneously, in another channel, deleted a durable queue with
  persistent messages involved in that tx, rabbit could terminate with
  an error.
- When a client was using both basic.qos and channel.flow, the latter
  would fail to re-enable message flow.
- When using 'confirm' mode, the deletion of queues could cause nacks
  to be issued (incorrectly).
- In extremely rare circumstances (never observed in the wild), a
  queue with a per-queue message ttl could break during sudden changes
  in rabbit memory usage.

2.6.0 enhancements
- Introduce active-active HA, with queues getting mirrored on nodes in
  a cluster. See http://www.rabbitmq.com/ha.html.
- Revamp the handling of AMQP's tx (transaction) class and clarify its
  behaviour See http://www.rabbitmq.com/specification.html#tx.
- Replace the 'administrator' flag, as used by the management plugin,
  with a more general 'user tags' mechanism. See
  http://www.rabbitmq.com/man/rabbitmqctl.1.man.html#set_user_tags.
- Do not require 'configure' permissions for passive queue/exchange
  declaration.
- Optimise of message delivery on channels with a basic.qos
  prefetch limit that are consuming from many queues.
- In 'rabbitmqctl list_channels', do not show the tx mode by default.
- When a cluster 'degrades' to only containing ram nodes - through
  'rabbitmqctl' actions or node failure - display/log a warning.
- Eliminate some spurious errors from the sasl log.

(fhajny)