get_children() should use get_posts()

[mdawaffe]

Attached makes get_children() more useful for plugins by going through get_posts().

1. Alter get_posts() to accept post_parent, post_type, post_status.
2. get_children() calls get_posts().

Please check to ensure backwards compatibility.

[ryan]

Looks good offhand. Will test.

[mdawaffe]

I guess the defaults should be empty strings for everything but post_parent to make it back compat.

[mdawaffe]

Back Compat defaults

[ryan]

(In [4503]) get_children() uses get_posts(). Props mdawaffe. fixes #3189