Rename "osname" → "stateroot"

I never really liked the term "osname". I feel "stateroot" is a *lot* clearer,
since the osname/stateroot mostly just holds `/var`. Further it avoids the `os`
prefix which is already overloaded.

Some of the existing docs already talked about "operating system state", which
further reinforces this.

There's *lot* more things than this which reference the term "osname", but I
don't want to change *everything* yet in this patch in case we decide to do
something different - this just gets the highlights.

Closes: #794
Approved by: jlebon

src/libostree/ostree-deployment.c

@@ -39,6 +39,12 @@ ostree_deployment_get_bootcsum (OstreeDeployment *self)
   return self->bootcsum;
 }
 
+/*
+ * ostree_deployment_get_osname:
+ * @self: Deployemnt
+ *
+ * Returns: The "stateroot" name, also known as an "osname"
+ */
 const char *
 ostree_deployment_get_osname (OstreeDeployment *self)
 {