Solution: accept only the mechanism defined by the socket options.

I've not tested this yet, so it's a speculative fix.

Solution: check that it rejects attempts to connect to a CURVE server
using NULL or PLAIN client.

Solution: change setsockopts on printable keys to expect 41, nor 40
bytes. Code still accepts 40 bytes for compatibility, and copies the
key to a well-terminated string before using it.

Fixes #1148

* Added new man page for this
* Added test case, in tests/test_security_curve.cpp
* Noted in zmq_utils.h that these methods are documented

This reverts commit bfd472f9.

This reverts commit 1206f457.

* ZAP handler thread was not getting time to start up
* Code now creates and binds handler socket in parent thread and
  passes the socket to the zap_handler, so this always gets the
  authentication requests.

* Removed redundant Z85 code and include files from project
* Simplified use of headers in test cases (now they all just use testutil.hpp)
* Export zmq_z85_encode() and zmq_z85_decode() in API
* Added man pages for these two functions

* 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.

* 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

- 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

- 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 :-)

adds expect_bounce_fail test function

which is like bounce, but fails if messages arrive.

This change adds the socket identity infomartion from the socket to the
zap frames.  In doing this the ZAP is able preform different operations
based on different sockets.  This is not compaitable with the current ZAP
RFC, but that can be updated.  As the ZAP rfc is currently draft for I
did not change the version number.

Tests also modified and passing.

The use of binary for CURVE keys is painful; you cannot easily copy
these in e.g. email, or use them directly in source code. There are
various encoding possibilities. Base16 and Base64 are not optimal.
Ascii85 is not safe for source (it generates quotes and escapes).

So, I've designed a new Base85 encoding, Z85, which is safe to use
in code and elsewhere, and I've modified libzmq to use this where
it also uses binary keys (in get/setsockopt).

Very simply, if you use a 32-byte value, it's Base256 (binary),
and if you use a 40-byte value, it's Base85 (Z85).

I've put the Z85 codec into z85_codec.hpp, it's not elegant C++
but it is minimal and it works. Feel free to rewrap as a real class
if this annoys you.

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

Test is skipped if no libsodium.
Added warning if libsodium not configured

- username and password sent as two string frames
- fixed test case to match

- we need to switch to PLAIN according to options.mechanism
- we need to catch case when both peers are as-server (or neither is)
- and to use username/password from options, for client

* ZMQ_PLAIN_SERVER, ZMQ_PLAIN_USERNAME, ZMQ_PLAIN_PASSWORD options
* Man page changes to zmq_setsockopt and zmq_getsockopt
* Man pages for ZMQ_NULL, ZMQ_PLAIN, and ZMQ_CURVE
* Test program test_security