Problems: DRAFT build broken in CentOS 6, missing bits in documentation

added note concerning issue 2788 to INSTALL

Solution: define __STDC_LIMIT_MACROS in test_timers.cpp before
including testutil.hpp so that the definitions are included

added libiphlpapi to PKGCFG_LIBS_PRIVATE for static mingw builds

Solution: print unsigned integers instead of size_t to avoid the error:

  error: ISO C++ does not support the 'z' gnu_printf length modifier

due to very old version of GCC.

Solution: mention that the STATISTICS command returns a message with 8
frames

Solution: fix it

Increase details in docs

* Add ZMQ_THREAD_NAME_PREFIX ctx option

Problem: Travis does not build with NORM

Solution: install package and enable the build on some Linux jobs

Add documentation for new steerable proxy command

Problems: autoconf does not use pkgconfig for NORM, deb packages do not build with libnorm

Solution: add dependency and configure flag

Solution: use the pkgconfig macro as the first step and then fallback
to manual checks

Problems: no documentation for new thread affinity and priority options, test_ctx_options only checks global DRAFT flag

Solution: add a section in zmq_ctx_set's manpage

Solution: mention that it is used on Linux when SCHED_OTHER is selected

Solution: this will break once the tested APIs move from DRAFT to STABLE
so instead check for the specific macros.

* Background thread scheduling 

- add ZMQ_THREAD_AFFINITY ctx option; set all thread scheduling options
from the context of the secondary thread instead of using the main
process thread context!
- change ZMQ_THREAD_PRIORITY to support setting NICE of the background
thread when using SCHED_OTHER

Add my RELICENSE

Problems: strict ZAP protocol adherence is backward incompatible, minor static analysis warnings

Solution: add ZMQ_ZAP_ENFORCE_DOMAIN to hide backward incompatible
change and make it disabled by default.
In a future release that breaks API compatibility we can then switch
the default to enabled in order to achieve full RFC compatibility.

Fixes #2762

Solution: remove unused zap_ipc_creds boolean variable

Solution: fix it

Solution: wrap bitwise ANDs in brackets as the static analyzer suggests

Problem: lcov autoconf macro out of date

Solution: update it

Solution: pre-create the second socket to save a few cycles between
closing the old one and binding the new one

Solution: name it after the test to avoid possible clashes when
running tests in parallel.

Solution: take a copy of the file descriptor before setting it to
retired_fd.

Problem: Race condition in IPC sockets

Solution: Don't unlink file on close

File may not belong to the socket at that point.