Add static compilation on CMake

Thread scheduling parameters: Use ZMQ context options instead of environment variables.

environment variables.

Code cleanup

Problem: two header files for a single library

Add missing socks files

socks: fix build on FreeBSD

The fix should be sane on all UNIX-like systems, so there's
no ZMQ_HAVE_FREEBSD involved.  It's likely that other BSDs
stumble across this problem too.

Solution: fixed this. Also minor code cleanups.

Users who need e.g. zmq_curve_keypair() have to remember to include
zmq_utils.h, which is counter-intuitive. The whole library should be
represented by a single include file.

Solution: merge all contents of zmq_utils.h into zmq.h, and deprecate
zmq_utils.h. Existing apps can continue unchanged. New apps can ignore
zmq_utils.h completely.

Problem: zmq_msg_gets did not set errno on unknown properties

Solution: set errno to EINVAL when a property does not exist.

Also fixed test_metadata.cpp to test this case.

Reverted removal of [ ] support 

This reverts commit 78a7b469.

Set EINVAL on invalid tcp:// endpoint in zmq_connect

Solution: set errno to EINVAL when tcp:// endpoint is invalid (was just
leaving errno to previous value).

Allow change of pthread priority

Rationale: In a real-time environment it is sometime mandatory to tune
threads priority and scheduling policy. This is required by our users
who mixes real-time and server threads within
the same process. It's not planned to support this on non-pthread
platforms (e.g. Windows).

Problem: zmq_connect() does not validate TCP addresses

This syntax is undocumented and has no known meaning. It was in libzmq
since 3.x.

Solution: remove this code.

Since https://github.com/zeromq/libzmq/commit/350a1a, TCP addresses
get resolved asynchronously, so zmq_connect no longer returned an
error on incorrect addresses.

This is troublesome since we rely on some error checking to catch
blatant errors.

Solution add some upfront syntax checking that catches at least the
obvious kinds of errors (invalid characters, wrong or missing port
number).

Fixed typo in doc example

Fix windows build

Updated spec file for libzmq 4.0.4

Added capabilities API

Fix memory leak in socks connecter

Add support for SOCKS proxies

This is still raw and experimental.
To connect through a SOCKS proxy, set ZMQ_SOCKS_PROXY socket option on
socket before issuing a connect call, e.g.:

    zmq_setsockopt (s, ZMQ_SOCKS_PROXY,
        "127.0.0.1:22222", strlen ("127.0.0.1:22222"));
    zmq_connect (s, "tcp://127.0.0.1:5555");

Known limitations:
- only SOCKS version 5 supported
- authentication not supported
- new option is still undocumented

Fixed build with arm-linux-androideabi toolchain