ssh/auth.c: Rename some procedures

* ssh/auth.c (guile_ssh_userauth_pubkey): Rename to
  `guile_ssh_userauth_public_key_x'.  All callers updated.
  (guile_ssh_userauth_pubkey_auto_x): Rename to
  `guile_ssh_userauth_public_key_auto_x'.  All callers updated.
  (guile_ssh_userauth_password): Rename to
  `guile_ssh_userauth_password_x'.  All callers updated.
  (guile_ssh_userauth_public_key_try): New procedure.
* ssh/auth.h, ssh/auth.scm: Update.
* examples/echo/client.scm.in (main): Update.
* examples/sssh.scm.in (main): Update.
* tests/client-server.scm ("userauth-pubkey!, success"): Rename to
  "userauth-public-key!, success".  Update.
* doc/api-auth.texi (Auth): Update.  Add description of
  `userauth-public-key/try' and `userauth-agent!' procedures.
* NEWS: Update.