Consider replacing `fm_sanitize_textarea()` with `sanitize_textarea_field()` (where available)

[dlh01]

WordPress 4.7 is on track to add a sanitizing function for textareas -- see https://core.trac.wordpress.org/changeset/38944.

It doesn't seem like fm_sanitize_textarea() could be removed entirely, but, if sanitize_textarea_field() makes it into 4.7, then FM could potentially defer to core when the function exists.