Put cycle-detecting code inside an

Put cycle-detecting code inside an
#ifdef ENABLE_CYCLE_CHECK, for systems lacking d_ino.
[D_INO_IN_DIRENT]: Define ENABLE_CYCLE_CHECK.

(ASSIGN_STRDUPA): Clean up.
(right_justify): Use memcpy return value directly.
Suggestions from Ulrich Drepper.

(remove_cwd_entries) [! defined HAVE_WORKING_READDIR]: Impose overhead
of working around non-POSIX readdir implementation only on systems
that need it.
(remove_cwd_entries): Temporarily save a copy of each entry name in
an obstack rather than on the stack via alloca, then free it in the
likely event that we don't save the entry in the hash table.