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