Authenticating SphinxAPI connections via a client_key setting.

Setting is for the searchd section of a configuration file, and only is used if set - otherwise, connections are handled normally with no authentication check.

Conflicts:
	src/sphinxutils.cpp
Reworking C code - this may work better on my Ubuntu server.
Don't check client key when opening a persistant connection.
Expect the client key when opening persistent connections.
Log requests with invalid client keys.