bug#18203: 24.4.50; `ffap-url-p' should NOT be a defsubst

[Drew Adams <drew.adams@oracle.com>]

> > There is NO reason for this to be a defsubst.  There is no reason for
> > users not to take advantage of this function generally.
> 
> I changed it to a defun,

Thank you.

> but I don't know what you mean by "There is no
> reason for users not to take advantage of this function generally.".
> You can use a defsubst just as you would a defun.

No, you cannot.

If existing code invokes a defsubst and you have only the compiled
version of that code, then you have no access to the function
defined by that defsubst.  The function _does not exist_ at runtime.
You cannot advise it or debug it, and redefining it has no effect.

Taking advantage of the function _"generally"_ includes having it
available at runtime - it requires its existence _as a function_.

`defsubst's are almost a misfeature nowadays.  In any case, they
be used should rarely, and only with good justification.