Declare lstat.

Declare lstat.
Rename change_symlinks member to `dereference' and use the DEREF_*
enum values.
(describe_change): Merge the chgrp and chown switch statements.
Use xmalloc to form the `user:group' string.
(change_file_owner): Record (and later, use) is_symlink and is_directory
from the lstat stats, in order to control whether we operate on symlinks
and whether (with -R) we traverse symlinks to directories.
When dereferencing, use open/fchown (rather than chown) on symlinks.