Loosen validation regex to use sanitize_key() in a few AJAX locations for...

Loosen validation regex to use sanitize_key() in a few AJAX locations for things like pages, orders, columns. Change return value to 0 for failure, as -1 is reserved for authentication/intention. props ocean90, azaozz, fixes #18637.

git-svn-id: https://core.svn.wordpress.org/trunk@18663 1a063a9b-81f0-0310-95a4-ce76da25c4cd