o Move userspecs, defaults and aliases into a new struct sudoers_parse_tree.

o Move userspecs, defaults and aliases into a new struct sudoers_parse_tree.
o The parse tree is now passed to the alias, match and defaults functions.
o The nss API has been changed so that the nss parse() function returns
  a pointer to a struct sudoers_parse_tree which will be filled in
  by the getdefs() and query() functions.