Lose create_function() in links_add_target() and links_add_base_url()....

Lose create_function() in links_add_target() and links_add_base_url(). esc_attr() the target to provide extra coverage for plugins.  Props Justin Rainbow, nacin. For trunk

git-svn-id: https://core.svn.wordpress.org/trunk@17459 1a063a9b-81f0-0310-95a4-ce76da25c4cd