Problem: does not initialize system before running selftest

This fails on Windows. Solution: call zsys_init() at start of
selftest method.

Fixes #542.
Problem: zsys_file_mode did not return RW attributes on Windows

There was an #ifdef UTYPE_ANDROID in the middle of the Windows
section, which makes no sense.

Solution: remove the #ifdef.

Fixes #543
Problem: initialization sequence was wrong

The sequence has to be:

1. get environment variables, initialize mutex and create socket
   reference list.
2. allow caller to set io_threads, max_sockets
3. create context and then sockets

So bundling (1) and (3) together did not work. Now the context is
created lazily at first zsys_socket(), and the _set methods all call
zsys_init().
Various fixes

Fixes #545
Fixes #544

More work on extended test case for zgossip.
Cosmetic style fixes
Added gossip tester
Updated man pages
Merge pull request #546 from hintjens/master

Problem: does not initialize system before running selftest