Generic module for handling SASL authentication & encryption

Generic module for handling SASL authentication & encryption

This provides two modules for handling SASL

 * virNetSASLContext provides the process-wide state, currently
   just a whitelist of usernames on the server and a one time
   library init call

 * virNetTLSSession provides the per-connection state, ie the
   SASL session itself. This also include APIs for providing
   data encryption/decryption once the session is established

* src/Makefile.am: Add to libvirt-net-rpc.la
* src/rpc/virnetsaslcontext.c, src/rpc/virnetsaslcontext.h: Generic
  SASL handling code