message.scm: Unify way of getting information from requests.

* src/message.scm: Unify way of getting information from requests.
  (auth-req:user, auth-req:password, auth-req:pubkey, pty-req:term)
  (pty-req:width, pty-req:height, pty-req:pxwidth)
  (pty-req:pxheight, env-req:name, env-req:value, exec-req:cmd)
  (global-req:addr, global-req:port): New procedures.
* src/message-func.c (get_auth_req, get_pty_req, get_env_req)
  (get_exec_req, get_global_req, guile_ssh_message_get_req): New
  procedures.
  (guile_ssh_message_auth_get_user)
  (guile_ssh_message_auth_get_password)
  (guile_ssh_message_auth_get_public_key)
  (guile_ssh_message_exec_get_command): Remove.