(struct alias): Add two new struct members: NAMESP and

(struct alias): Add two new struct members: NAMESP and
ALIASEE to help work with namespace aliases.
(struct sym): Remove struct member NAMESP_ALIASES.
(namespace_alias_table): New variable.
(make_namespace): Add parameter CONTEXT.
(check_namespace): New function.
(find_namespace): Add parameter CONTEXT.
(check_namespace_alias): New function.
(register_namespace_alias): Change type of parameter
OLD_NAME. Search for already defined alias in
NAMESPACE_ALIAS_TABLE.
(check_namespace): New function.
(enter_namespace): Call find_namespace with CONTEXT parameter.
(match_qualified_namespace_alias): New function.
(parse_qualified_ident_or_type): Fixed typo in comment. While
parsing qualified ident or type update namespace context and
restore it on exit.
(parse_qualified_param_ident_or_type): Fixed typo in comment.
(globals): Changed handling of namespace aliases.
(version): Added year 2001.