src/auth.{c,h,scm}: Implement ssh:userauth-get-list.

Implement ssh:userauth-get-list that returns a list of available
authentication methods for a given SSH session.
* src/auth.c (guile_ssh_userauth_get_list): New function.
  (init_auth_func): Define ssh:userauth-get-list.
* src/auth.h: Likewise.
* src/auth.scm: Add ssh:userauth-get-list.