When unregistering an I/O callback from a virNetSocket object,
there is still a chance that an event may come in on the callback.
In this case it is possible that the virNetSocket might have been
freed already. Make use of a virFreeCallback when registering
the I/O callbacks and hold a reference for the entire time the
callback is set.

* src/rpc/virnetsocket.c: Register a free function for the
  file handle watch
* src/rpc/virnetsocket.h, src/rpc/virnetserverservice.c,
  src/rpc/virnetserverclient.c, src/rpc/virnetclient.c: Add
  a free function for the socket I/O watches

Remove the need for a virNetSocket object to be protected by
locks from the object using it, by introducing its own native
locking and reference counting

* src/rpc/virnetsocket.c: Add locking & reference counting

Rather than trying to clean up the ssh child ourselves, and risk
subtle differences from the socket creation error path, we can
just use the new APIs.

* src/rpc/virnetsocket.c (virNetSocketFree): Use new function.

Set StrictHostKeyChecking=no to auto-accept new ssh host keys if the
no_verify extra parameter was specified.  This won't disable host key
checking for already known hosts.  Includes a test and documentation.

Detected by Coverity.  Both are instances of bad things happening
if pipe2 fails; the virNetClientNew failure could free garbage,
and virNetSocketNewConnectCommand could close random fds.

Note: POSIX doesn't guarantee the contents of fd[0] and fd[1]
after pipe failure: http://austingroupbugs.net/view.php?id=467
We may need to introduce a virPipe2 wrapper that guarantees
that on pipe failure, the fds are explicitly set to -1, rather
than our current state of assuming the fds are unchanged from
their value prior to the failed pipe call.

* src/rpc/virnetclient.c (virNetClientNew): Initialize variable.
* src/rpc/virnetsocket.c (virNetSocketNewConnectCommand):
Likewise.

virNetSocketFree uses kill(SIGTERM) so we must include
signal.h for the definitions

* src/rpc/virnetsocket.c: Include signal.h

The RPC client treats failure to register a socket watch
as non-fatal, since we do not mandate that a libvirt client
application provide an event loop implementation. It is
thus inappropriate to a log a message at VIR_LOG_WARN

* src/rpc/virnetsocket.c: Lower logging level

Allow RPC servers to advertise themselves using MDNS,
via Avahi

* src/rpc/virnetserver.c, src/rpc/virnetserver.h: Allow
  registration of MDNS services via avahi
* src/rpc/virnetserverservice.c, src/rpc/virnetserverservice.h: Add
  API to fetch the listen port number
* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Add API to
  fetch the local port number
* src/rpc/virnetservermdns.c, src/rpc/virnetservermdns.h: Represent
  an MDNS advertisement

This extends the basic virNetSocket APIs to allow them to have
a handle to the TLS/SASL session objects, once established.
This ensures that any data reads/writes are automagically
passed through the TLS/SASL encryption layers if required.

* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Wire up
  SASL/TLS encryption

Introduces a simple wrapper around the raw POSIX sockets APIs
and name resolution APIs. Allows for easy creation of client
and server sockets with correct usage of name resolution APIs
for protocol agnostic socket setup.

It can listen for UNIX and TCP stream sockets.

It can connect to UNIX, TCP streams directly, or indirectly
to UNIX sockets via an SSH tunnel or external command

* src/Makefile.am: Add to libvirt-net-rpc.la
* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Generic
  sockets APIs
* tests/Makefile.am: Add socket test
* tests/virnetsockettest.c: New test case
* tests/testutils.c: Avoid overriding LIBVIRT_DEBUG settings
* tests/ssh.c: Dumb helper program for SSH tunnelling tests