OstreeSePolicy: add ostree_sepolicy_get_csum()

This can be used as a fingerprint to determine whether two
OstreeSePolicy objects are equivalent.

Also add documentation for ostree_sepolicy_get_name().

Closes: #219
Approved by: cgwalters

src/libostree/ostree-sepolicy.h

@@ -44,6 +44,9 @@ GFile * ostree_sepolicy_get_path (OstreeSePolicy  *self);
 _OSTREE_PUBLIC
 const char *ostree_sepolicy_get_name (OstreeSePolicy *self);
 
+_OSTREE_PUBLIC
+const char *ostree_sepolicy_get_csum (OstreeSePolicy *self);
+
 _OSTREE_PUBLIC
 gboolean ostree_sepolicy_get_label (OstreeSePolicy    *self,
                                     const char       *relpath,