ssh/auth.c: Remove username form parameter list of functions.

* ssh/auth.c (guile_ssh_userauth_pubkey): Don't take a username as
  an parameter.  All callers updated.
  (guile_ssh_userauth_password): Likewise.
* examples/ssshd.scm.in (handle-req-auth): Update.
* examples/echo/client.scm.in (main): Update.
* tests/client-server.scm ("userauth-password!, success")
  ("userauth-password!, denied", "userauth-password!, partial")
  ("userauth-pubkey!, success"): Update TCs.
* doc/api-auth.texi (Auth): Update documentation for
  `userauth-pubkey!' and `userauth-password!'.  Add a general note
  about setting of a username.
* doc/version.texi: Update.