src/server.scm: Use `server-set!' for setting blocking/nonblocking mode.

Unificate the way of server configuration: use `server-set!' to set
server to the blocking/nonblocking mode.

* src/server-func.c (set_option): Handle 'blocking-mode
  (guile_ssh_server_set_blocking_x): Remove procedure.
* src/server.scm (server-set-blocking!): Remove procedure.
  (make-server): Handle "blocking-mode" keyword.