Remove xstat function pointer member. The way it was used was not

Remove xstat function pointer member.  The way it was used was not
portable, since some systems (OSF V5.1, Solaris 2.5.1) provide static
inline `stat' and `lstat' functions, thus making the tests of
`xstat == lstat' in copy.c always fail.

(copy_dir): Set `.dereference' member, not .xstat.
(copy_internal): Use `XSTAT (x, ...)' in place of `*(x->xstat) (...)'.
Use `x->dereference == DEREF_NEVER' in place of `x->xstat == lstat'.
(valid_options): Remove now-obsolete FIXME comments.