(QCvars): New variable.

(QCvars): New variable.
(syms_of_process): Intern and staticpro it.
(Fprocess_variable, Fset_process_variable): New functions.
(syms_of_process): Defsubr them.
(Fstart_process): Initialize private_vars plist to nil.
(Fmake_network_process): New arg :vars to setup the private
variables for new network process.
(server_accept_connection): Copy server's private variables to
client process.