cp: omit unnecessary stat of destination

cp: omit unnecessary stat of destination

'cp A B' attempts to open B as a directory, to see whether to
write to B/A instead of to B.  In the common case where the
open fails with ENOENT, do not bother to stat B afterwards
since the stat should also fail with ENOENT.
* src/copy.c (copy_internal, copy): Change bool arg about
nonexistent destination to a 3-way int argument.  All callers changed.
(copy_internal): Do not bother to stat a destination already known
to not exist when following symlinks.