Manual and welcome documentation

remove debug printf in test_xpub_welcome_msg

Manual and welcome

Fix issue #1257

Fix test_filter_ipc for cleared supplementary groups

This should fix part of [https://github.com/zeromq/libzmq/issues/1129].

Problem: linger values other than -1 or 0 are unsafe

Solution: set defaults back to infinity, and add new context
option, ZMQ_BLOCKY that the user can set to false to get a
less surprising behavior on context termination. Eg.

    zmq_ctx_set (ctx, ZMQ_BLOCKY, false);

Fix busy-polling when reaching the rate limit

use more conservative 30 second default timeout for LINGER

Regression in #1248 can cause lost messages at exit.

Problem: return code of sodium_init() is not checked.

There are two todo comments in curve_client.cpp and curve_server.cpp that suggest
checking the return code of sodium_init() call. sodium_init() returns -1 on error,
0 on success and 1 if it has been called before and is already initalized:
https://github.com/jedisct1/libsodium/blob/master/src/libsodium/sodium/core.c

Use consistent spelling, fix typos.

behavior -> behaviour
initialize -> initialse
initialization -> initialisation

Problem: default LINGER value is insane

No error-checking of setsockopt ZMQ_CURVE_* z85 keys. Solves #1094.

Solution: use a sane value, e.g. 2 seconds

Fixes #1247

Documentation fix for zmq_ctx_term

Solution: document this with a clear warning. It would be
nicer perhaps to change the default LINGER to e.g. a few
seconds. However this could break existing applications.

Fix typo: addres -> address

Problem: qt-android build doesn't fully work for downstream builds

Solution: Fix the qt-android build

Restore structure to targets lost during nonrecursive refactor.