talloc: rename to libtalloc and cleanup dependency mess

To avoid errors when building dev-images (talloc-dev is missing), and to avoid
insane empty packages for that, rename talloc -> libtalloc as libtevent shows.

With that, remove dependencies to attr/xattr - unless explicitely enabled via
PACKAGECONFIG.

Signed-off-by: Jens Rehsack <sno@netbsd.org>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>