Get rid of root group defaults. Merge trees into expressions. Allow host statemen...

Get rid of root group defaults.   Merge trees into expressions.   Allow host statements in class declarations (explicitly declaring a host to be in a specific class).   Server universe declarations handled as statements.   parse_allow_deny now creates statements.      Grandfather old-style user and vendor class declarations.   Add support for spawning classes.      Can't flag dynamic bootp anymore.   Move parse_ip_addr_or_hostname into common code.   tree_cache -> option_cache.   Move option parameter parsing into common parser.   Put expression parser in common code.