When creating a virIdentityPtr for the system identity, include
the current process ID as an attribute.

There are various methods named "virXXXXSecurityContext",
which are specific to SELinux. Rename them all to
"virXXXXSELinuxContext". They will still raise errors at
runtime if SELinux is not compiled in
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Some code mistakenly called virIdentityOnceInit directly
instead of virIdentityInitialize(). This meant that one-time
initializer was run many times with predictably bad results.

The virNetSocket & virIdentity classes accidentally got some
conditionals using HAVE_SELINUX instead of WITH_SELINUX.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

If no user identity is available, some operations may wish to
use the system identity. ie the identity of the current process
itself. Add an API to get such an identity.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

To allow any internal API to get the current identity, add APIs
to associate a virIdentityPtr with the current thread, via a
thread local
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Introduce a local object virIdentity for managing security
attributes used to form a client application's identity.
Instances of this object are intended to be used as if they
were immutable, once created & populated with attributes
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>