r1200: Add 'gensec', our generic security layer.

r1200: Add 'gensec', our generic security layer.

This layer is used for DCERPC security, as well as ntlm_auth at this
time.  It expect things like SASL and the CIFS layer to use it as
well.

The particular purpose of this layer is to introduce SPENGO, which
needs generic access to the actual implementation mechanisms.

Schannel, due to it's 'interesting' setup properties is in GENSEC, but
is only in the RPC code.

Andrew Bartlett