NULL: Implement ERROR handling

NULL mechanism sends ERROR command rather than READY command when ZAP
handler rejects the ZAP request (status code != "200"). The body of
ERROR command contains the status code as returned by ZAP handler.

Integrate GSSAPI option in VS UI options and props.

Update mechanism API so we can check for ERROR status

ipc: fail harder for abstract ipc on non-Linux

Using 'ipc://@abstract-socket' on non-Linux platforms yields inconsistent
behaviour.  Abstract sockets don't exist, so the literal file is created.
The test previously failed, but for a different reason: this is not the
directory you are looking for.  Now, zmq_bind() will fail for the right
reason: the socket can't be created.  Put the XFAIL back.

tests: fix abstract ipc test by omitting slashes

gssapi: RFC 2744 mandates GSS_C_NT_HOSTBASED_SERVICE

Tested on FreeBSD 10.

Minor renaming

Build-system: disable test_fork when fork() is not available

fork() support is optional and its availability is correctly detected at
contfigure time.

But test_fork was all always built, preventing build for targets that do
not provide fork() from building successfully.

This pacth fixes the CMakeLists.txt on this point.

fork() support is optional and its availability is correctly detected at
contfigure time.

But test_fork was all always built, preventing build for targets that do
not provide fork() from building successfully.

This pacth fixes the autotools on this point.

Remove i_properties interface

We use metadata_t directly. No need for generic interface now.

Initialise _has_src_addr variable, fixes hang on Windows.

Added test_metadata

Solution: add this to ZAP reply processing.

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

These commits broke regression tests, so I'm reverting them.

This reverts commit 5e0facda.

This reverts commit 2a84d259.

Broke regression test on test_hwm.cpp.

Feature/divide by zero investigation

These changes cause an assertion in lb_sendpipe() when executing the test_security_plain testcase.

Provides control of race windows and logging for some of the interesting transitions.

Include ZMTP properties in message metadata