embed.fnc: Change fcn from A to X

embed.fnc: Change fcn from A to X

This function is marked as accessible anywhere, but experimental, and
so is changeable at any time without notice, and its name begins with an
underscore to indicate its private nature.  I didn't know at the time I
wrote it that we have an existing mechanism to deal with functions whose
only use should be a public macro.  This changes to use that mechanism.