Fix invalid address metadata for ZMQ_EVENT_DISCONNECTED

Fix issue #406

When a peer reconnects, the router socket receives an identity
message containing this peer id. When this happens, the current
implementation crashes.

This patch makes a router socket to silently ignore all identity
messages coming from reconnected peers.

Fix LIBZMQ-404: zmq_term not truly re-entrant

zmq_term can not safely be re-entered with pgm transport.
Fix proposed by Steven McCoy.

Correction for LIBZMQ-335

That way it can be used more appropriately between processes.

By assigning a SECURITY_DESCRIPTOR to the event we gain the ability to
share it between service and console programs. We also added
EVENT_MODIFY_STATE as a requirement to OpenEvent so we can SetEvent later
in the method.

Small code cleanup

Fix issue #397

When closing an ipc listener, the library may try to unlink
the associated file. When this fails, the underlying
socket is not marked as retired and this triggers
assertion failure.

Fixes issue #397

Be more conservative about when to generate ZMQ_EVENT_CLOSE_FAILED

This also fixes a bug in tcp_connecter and tcp_listener, which
generated the event not when they failed to close the socket but
when the succeed to close it.

Small cleanup

Rewrite event processing in io_thread

Code cleanup

Reverting fix for #393

Fixed issue #393 - Android build error

Fixed HWM doc on ZMQ_PULL

Making Android&pyzmq friendly version with -avoid-version

mtrie clenaup

Let docs for zmq_ctx_set_monitor() respect a 80 char wide column width

Small code simplification

Fix a bug in trie implementation

When removing keys for a specified value, make sure we drop
empty node table. Failing to do this can lead to asserion failure.

Refs: http://lists.zeromq.org/pipermail/zeromq-dev/2012-June/017589.html

declare struct iovec