Add tests for SOCKS proxy support

problem: no artifacts for x64 on windows

solution: add x64 builds to appveyor

Solution: don't run tests on tags

This doesn't break anything, as tags was not built until now. Anyway, when building tags we really care about the release of the packages and not tests

Problem: test assertions not reporting the right line number

Solution: pass the line number as part of the testutil functions

problem: appveyor with libsodium change the current directory and bre…

Solution: do git pull without changing the current directoy

problem: no artifact repository for windows

Solution: deploy artifcats to bintray

Problem: formatting errors

Solution: copy it from PR https://github.com/zeromq/libzmq/pull/3537

Solution: apply clang-format-diff

pthread_equal to compare thread ids

I am trying to port ZeroMQ to z/OS USS and I faced several problems. One of them is this function "is_current_thread" trying to compare two thread ids with operator "==". I've changed the code to use the official function "pthread_equal" to compare thread ids and now it's working fine.

Implement SOCKS5 basic authentication

Fix issues with SOCKS5 proxy connection

Two issues where introduced by commit 12c4b55a:
- the proxy connection was done to the target address instead of
the proxy address
- on error the proxy connection status was not reset to unplugged

Problem: Unclear `zmq_msg_send` failure semantics

Solution: Explicitly explain message ownership semantics when the call
fails.

Fixes #3526.

Add missing information on multicasting with high data rates to PGM docs

Solution: Include a new section on configuring both ZMQ and the host
OS tx/rx buffers to facilitate sending large messages at a high data
rate with the PGM protocol.

Solution: Added relicense agreement

acinclude.m4, configure.ac: Fix several non-portable uses of `test`

Fixes #3517

Fix Fedora 30 build

Implement thread_t::setThreadName() on windows

Problem: clang-format 5 has issues with comments

Problems: typos in docs, unnecessary ifdefs, duplicated APIs

Solution: now that clang-format 6+ is used in the CI, fix it

Solution: bump CI to version 7

Solution: unify the two APIs, as they are both still in DRAFT state and
thus can be changed.

Solution: fix them

Solution: use unlikely to optimize for the most common case