Problem: two header files for a single library

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: missing stdlib.h include in curve_keygen.cpp

Solution: fixed this. Also minor code cleanups.
Merge pull request #1112 from hintjens/master

Problem: two header files for a single library
Merge pull request #1115 from hurtonm/master

Code cleanup
Thread scheduling parameters: Use ZMQ context options instead of
environment variables.
Merge pull request #1117 from jlauenercern/master

Thread scheduling parameters: Use ZMQ context options instead of environment variables.
Add static compilation on CMake
Merge pull request #1118 from guidefloripa/master

Add static compilation on CMake
Fix to build on OpenBSD
Merge remote-tracking branch 'upstream/master'
duplicated type definitions when using CMAKE
Merge pull request #1119 from mosconi/master

Fix to build on OpenBSD
Don't delay reception of signal

- new code may help undersdtand issue #1108
  (https://github.com/zeromq/libzmq/issues/1108)
- code cleanups