server-func.c (server-accept!): Improve.

* src/server-func.c (guile_ssh_server_accept_x): Rename to
  `guile_ssh_server_accept'.  Change Scheme name to `server-accept'.
  Change the function so it has no side effects.
* src/server-func.h: Update.
* src/server.scm: Export `server-accept'.
Rename `server-listen' to `server-listen!' because of side effects.

* src/server-func.c (guile_ssh_server_listen): Rename to
  `guile_ssh_server_listen_x' because of side effects.  Change Scheme
  name to `server-listen!'.
* src/server.scm: Rename `server-listen' to `server-listen!'.