Add IBM i build support

problem: unsecured websocket is rarely used in production

Solution: support websocket with tls (wss)

Add grant by MongoDB, Inc.

Received by Github comment:
https://github.com/zeromq/libzmq/pull/1451#issuecomment-537169373

Add relicense grant

Fix behavior of ZMQ_CONFLATE on PUB sockets

Solution: Fix behavior of dbuffer on writes. Message passed in argument
of dbuffer::write can be assigned directly to the back buffer without
leaking, since the message has already the right reference count (see
dist::distribute and msg_t::add_refs). Secondly, in order to prevent the
message pending in the front buffer from leaking if it hasn't been
closed yet (for example because the peer is not reachable anymore), move
back buffer to front buffer using msg_t::move instead of swapping the
buffers, thus allowing release of the message in the front buffer.

Add relicense grant

Fix documentation about sockets blocking on send operations

Solution: Reflect real behavior of sockets blocking on send operations
Sockets that block in mute state only block when no peer is available if
the transport is connection-oriented and the ZMQ_IMMEDIATE socket option
is set.

Allow "configure --disable-maintainer-mode"

Allow "configure --disable-maintainer-mode" to disable timestamp checking.
This is useful for one-off builds, in particular on e.g. clusters, where slightly skew clocks force aclocal and friends to kick in for no good reason.

Bugfix/dont include winsock2

Solution: don't include winsock2.h, replace its only use (reference to SOCKET) by explicitly naming underlying type.

Problem: nss package has different name on SUSE

Solution: fix it in the spec file

Problems: packages not building with nss, build fails on SUSE Tumbleweed, too many Travis builds

Solution: add -ffat-lto-objects if the compiler supports it

Solution: do not add one specifically for nss, but use the existing
catch-all build

Solution: add relevant dependencies

Solution: only do it once if the macro is enabled

problem: sha external file conflict with czmq

Solution: add an nss build to CI

Solution: allow to use external library (nss) for sha1 to avoid the conflict

RELICENSE: add grant from Stefan Herbrechtsmeier

Relicense: Jean-Christophe Fillion-Robin

Relicense granby by RPGillespie6

Received by Github comment:
https://github.com/zeromq/libzmq/pull/2536#issuecomment-528961084

problem: ws_engine doesn't check Sec-WebSocket-Protocol

Solution: Validate Sec-WebSocket-Protocol and make sure ZWS2.0 is one of the protocols

problem: ws_engine doesn't send correct host and path