add --preserve= support

[daltonbohning]

Some tools (dcp, dsync) have the option --preserve to preserve ownership, permissions, acls, timestamps, and xattrs. Currently, this option is all or nothing. It would be useful to enhance this with finer controls:
--preserve[=ATTR_LIST]
This would allow specific subsets to be copied.

[akesandgren]

See also #418 which is done more in the style of rsync.

[ofaaland]

@daltonbohning take a look at this commit which provides that kind of control through /etc/xattr.conf (libattr):

• 96a9fe7 dsync and dcp: allow user to control copying of xattrs