(_GNU_SOURCE): Remove; autoconf now does this.

 (_GNU_SOURCE): Remove; autoconf now does this.
Include <hash.h>.
(getpwuid, getgrgid): Declare only if system headers don't.
(gid_to_gname): Don't invoke setgrent.
(namelist): Now static, not global.
(nametail): New var.  All uses of namelast changed to use nametail,
with one extra level of indirection.
(name_gather): Use memcpy instead of strncpy + assignment of NUL.
(name_match): Set nametail too, when setting namelist to null.
(add_hierarchy_to_namelist): Change type of dir arg from char * to
struct name *, so that we don't have to look up the name again here.
Get change_dir from dir rather than as a separate arg.  Add dirsize arg,
and pass it along to get_directory_contents.  Remove unnecessary check
of directory type.
(new_name): Do not append a slash if PATH already ends in one.
(avoided_names, struct avoided_name): Remove.
(avoided_name_table): New var, replacing avoided_names.
(hash_avoided_name, compare_avoided_names): New function.
(add_avoided_name, is_avoided_name): Use hash table rather than linked list.