Instead of requiring the caller to format to/from strings,
add typesafe APIs todo this work.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Future improvements to the polkit code will require access to
the numeric user ID, not merely user name.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Since PIDs can be reused, polkit prefers to be given
a (PID,start time) pair. If given a PID on its own,
it will attempt to lookup the start time in /proc/pid/stat,
though this is subject to races.

It is safer if the client app resolves the PID start
time itself, because as long as the app has the client
socket open, the client PID won't be reused.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

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>

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>