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);
Merge pull request #1258 from hintjens/master

Problem: linger values other than -1 or 0 are unsafe
Fix test_filter_ipc for cleared supplementary groups

This should fix part of [https://github.com/zeromq/libzmq/issues/1129].
Merge pull request #1260 from abbradar/master

Fix test_filter_ipc for cleared supplementary groups
Fix issue #1257
Merge pull request #1262 from hurtonm/master

Fix issue #1257
Add manual control over subscriptions to Pub
xpub welcome msg
Merge pull request #1267 from somdoron/manual_and_welcome

Manual and welcome
remove debug printf in test_xpub_welcome_msg
Merge pull request #1268 from somdoron/manual_and_welcome

remove debug printf in test_xpub_welcome_msg