Make the OpenSSL locking function warnings more severe

Our git_openssl_set_locking() would ideally not exist. Make it clearer
that we provide it as a last resort and you should prefer anything else.
Add the OpenSSL  changes to the CHANGELOG

It seems these were forgotten when initially splitting this up.
Mention the init function in the README

This fixes #2731.
local: add failing test for sideband information

We do not currently generate any messages when we're counting the
objects, as might be expected from a local upload-pack. Assert that we
do call the function when working.
local: send 'counting objects' output

Pretend we have a git process at the other end by creating a similar
progress output when inserting objects into the packbuilder.
Fix public header on sys/refs.h

GIT_BEGIN/END_DECL were missing from sys/refs.h and preventing
compilation with g++ as the symbol were mangled.
COM0 is a valid path, although Windows Explorer does not allow to create this
Add more Windows reserved filenames
Merge pull request #2768 from dprofeta/fix/c++-refs

Fix public header on sys/refs.h
Merge pull request #2767 from linquize/winpath

More Windows Path checking
Merge pull request #2763 from libgit2/cmn/local-proto-progress

Show progress output on fetch for the local transport
Merge pull request #2760 from libgit2/cmn/init-readme

Mention the init function in the README
Merge pull request #2759 from libgit2/cmn/openssl-sys

Make OpenSSL locking warnings more severe