This patch adds a "secret" as a separately managed object, using a
special-purpose API to transfer the secret values between nodes and
libvirt users.

* docs/schemas/secret.rng, docs/schemas/Makefilem.am: Add new
  schema for virSecret objects
* docs/*html: Re-generated
* docs/formatsecret.html.in, docs/sitemap.html.in: Add page
  describing the virSecret XML schema
* include/libvirt/libvirt.h.in: Define the new virSecret public
  API
* src/libvirt_public.syms: Export symbols for new public APIs
* mingw32-libvirt.spec.in, libvirt.spec.in: Add secret.rng to
  files list