Change ZMQ_THREAD_AFFINITY to ZMQ_THREAD_AFFINITY_CPU_ADD/ZMQ_THREAD_AFFINITY_CPU_REMOVE. Avoid prefix thread names when no prefix was set.

Partial Windows XP support

Problem: one missed optimization opportunity for blob_t map lookup

Solution: create referencing blob_t

Problem: use of std::map::insert is inefficient

Solution: use std::map::emplace instead, where available

Problem: description of return value is cloned from zmq_atomic_counter_new

Solution: provide correct description

Fixes #2789

* Problem: excessive memory allocations around blob_t

Solution: redefine blob_t as a custom type, and use reference/move
semantics where possible

* Clarify usage of zmq_close

Problem: comment before sndtimeo/rcvtimeo does not specify dimension

Solution: add "milliseconds"

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

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