yang: embed models into binaries

yang: embed models into binaries

This bakes our YANG models straight into the library/daemons, so they
don't need to be loaded from /usr/share/yang.  This makes the
installation quite a bit more robust, as well as gets us halfway to
running uninstalled.  (The other half is baking in the extension type
module.)

The /usr/share/yang directory is still searched as a fallback, as well
as for the experimental YANG model translator.  This is likely to stay
as is for the time being.

Signed-off-by: David Lamparter <equinox@diac24.net>