Implement non-blocking shutdown command

Fixed minor error in NULL mechanism (spec was wrong)

* This is passed to the ZAP handler in the 'domain' field

* If not set, or empty, then NULL security does not call the ZAP handler

* This resolves the phantom ZAP request syndrome seen with sockets where
  security was never intended (e.g. in test cases)

* This means if you install a ZAP handler, it will not get any requests
  for new connections until you take some explicit action, which can be
  setting a username/password for PLAIN, a key for CURVE, or the domain
  for NULL.

Fix tests on windows

Updated libzmq to match RFCs 

- was not looping to read all input from peer
- broke on OS/X specifically

Clarified zmq_socket.txt ZMQ_STREAM section, added HTTP server example

- tests that system can provide at least 1,000 sockets
- we could expand on this but this covers the main case of OS/X
  having a too-low default limit of 256 handles per process

mark test_linger as expected failure

rather than pretending it passes 

* Command names changed from null terminated to length-specified
* Command frames use the correct flag (bit 2)
* test_stream acts as test case for command frames
* Some code cleanups

Fix MSVC 2008 and 2010 project files

Fix cmake file for security tests

Fixed ZAP authentication

- if ZAP server returns anything except 200, connection is closed
- all security tests now pass correctly
- test_security_curve now does proper client key authentication using test key
- test_security_plain now does proper password authentication

Fixed regression tests

- Split off NULL security check from PLAIN
- Cleaned up test_linger code a little
- Got all tests to pass, added TODOs for outstanding issues
- Added ZAP authentication for NULL test case
- NULL mechanism was not passing server identity - fixed
- cleaned up test_security_plain and removed option double-checks (made code ugly)
- lowered timeout on expect_bounce_fail to 150 msec to speed up checks
- removed all sleeps from test_fork and simplified code (it still passes :-)

add test_linger

Adding ability for a context to be terminated in a forked child process

Remove delay options

avoids -Wall failures comparing int to uint

test failed auth for both PLAIN and CURVE